feat: mobile-responsive mail plugin UI

frontend/src/components/layout/PluginToolbar.tsx

@@ -26,9 +26,9 @@ function ToolbarSearch({ item }: { item: ToolbarItem }) {
       type="text"
       placeholder={item.searchPlaceholder || 'Suchen...'}
       onChange={(e) => item.onSearch?.(e.target.value)}
-      className="w-40 px-2 py-1 text-xs border border-secondary-200 rounded
+      className="w-28 sm:w-40 px-2 py-1 text-xs border border-secondary-200 rounded
         focus:outline-none focus:border-primary-400 focus:ring-1 focus:ring-primary-300
-        placeholder:text-secondary-400"
+        placeholder:text-secondary-400 flex-shrink-0"
     />
   );
 }
@@ -38,9 +38,9 @@ function ToolbarSelect({ item }: { item: ToolbarItem }) {
     <select
       value={item.selectValue || ''}
       onChange={(e) => item.onSelect?.(e.target.value)}
-      className="px-2 py-1 text-xs border border-secondary-200 rounded
+      className="px-1.5 sm:px-2 py-1 text-xs border border-secondary-200 rounded
         focus:outline-none focus:border-primary-400 focus:ring-1 focus:ring-primary-300
-        bg-white cursor-pointer"
+        bg-white cursor-pointer flex-shrink-0 max-w-[120px] sm:max-w-none"
     >
       {item.selectOptions?.map((opt) => (
         <option key={opt.value} value={opt.value}>{opt.label}</option>
@@ -66,7 +66,7 @@ export function PluginToolbar() {
 
   return (
     <div
-      className="flex items-center gap-1 px-3 py-1 bg-white border-b border-secondary-200 min-h-[36px]"
+      className="flex items-center gap-1 px-2 sm:px-3 py-1 bg-white border-b border-secondary-200 min-h-[36px] overflow-x-auto"
       data-testid="plugin-toolbar"
     >
       {groupNames.map((groupName, gi) => (

frontend/src/components/mail/ComposeModal.tsx

@@ -151,7 +151,7 @@ export function ComposeModal({
   const title = mode === 'reply' ? t('mail.reply') : mode === 'forward' ? t('mail.forward') : t('mail.compose');
 
   return (
-    <Modal open={open} onClose={onClose} title={title} size="xl" closeOnBackdrop={false}>
+    <Modal open={open} onClose={onClose} title={title} size="xl" closeOnBackdrop={false} fullScreenMobile>
       <div className="space-y-4" data-testid="compose-modal">
         {/* Toolbar */}
         <div className="flex items-center gap-1 border-b border-secondary-200 pb-2" data-testid="compose-toolbar">
@@ -225,7 +225,7 @@ export function ComposeModal({
                 {t('mail.showCcBcc')}
               </button>
             ) : (
-              <div className="w-full grid grid-cols-1 md:grid-cols-2 gap-2">
+              <div className="w-full grid grid-cols-1 sm:grid-cols-2 gap-2">
                 <Input
                   label={t('mail.cc')}
                   value={cc}
@@ -278,7 +278,7 @@ export function ComposeModal({
         />
 
         {/* Actions */}
-        <div className="flex justify-end gap-2 pt-2 border-t border-secondary-200">
+        <div className="flex justify-end gap-2 pt-2 border-t border-secondary-200 sticky bottom-0 bg-white py-3">
           <Button variant="secondary" onClick={onClose} type="button">
             {t('common.cancel')}
           </Button>

frontend/src/components/mail/MailDetail.tsx

@@ -91,7 +91,7 @@ export function MailDetail({
   return (
     <div className="flex flex-col h-full" data-testid="mail-detail">
       {/* Toolbar */}
-      <div className="flex items-center gap-2 px-4 py-3 border-b border-secondary-200" data-testid="mail-detail-toolbar">
+      <div className="flex items-center gap-1 sm:gap-2 px-3 py-2 md:px-4 md:py-3 border-b border-secondary-200 overflow-x-auto" data-testid="mail-detail-toolbar">
         <Button variant="secondary" size="sm" onClick={() => onReply(mail)} icon={
           <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
             <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 10h10a8 8 0 018 8v2M3 10l6 6m-6-6l6-6" />
@@ -135,9 +135,9 @@ export function MailDetail({
       </div>
 
       {/* Headers */}
-      <div className="px-4 py-3 border-b border-secondary-200" data-testid="mail-detail-headers">
-        <div className="flex items-start justify-between gap-4">
-          <h2 className="text-lg font-semibold text-secondary-900 flex-1">{mail.subject || t('mail.noSubject')}</h2>
+      <div className="px-3 py-2 md:px-4 md:py-3 border-b border-secondary-200" data-testid="mail-detail-headers">
+        <div className="flex flex-col sm:flex-row sm:items-start sm:justify-between gap-2 sm:gap-4">
+          <h2 className="text-base md:text-lg font-semibold text-secondary-900 flex-1">{mail.subject || t('mail.noSubject')}</h2>
           {mail.labels && mail.labels.length > 0 && (
             <div className="flex gap-1 flex-shrink-0">
               {mail.labels.map((label) => (
@@ -148,30 +148,30 @@ export function MailDetail({
             </div>
           )}
         </div>
-        <div className="mt-2 space-y-1 text-sm text-secondary-600">
-          <div className="flex gap-2">
-            <span className="font-medium text-secondary-500">{t('mail.from')}:</span>
-            <span>{mail.from_name ? `${mail.from_name} <${mail.from_address}>` : mail.from_address}</span>
+        <div className="mt-2 space-y-1 text-xs md:text-sm text-secondary-600">
+          <div className="flex flex-col sm:flex-row sm:gap-2">
+            <span className="font-medium text-secondary-500 flex-shrink-0">{t('mail.from')}:</span>
+            <span className="break-words">{mail.from_name ? `${mail.from_name} <${mail.from_address}>` : mail.from_address}</span>
           </div>
-          <div className="flex gap-2">
-            <span className="font-medium text-secondary-500">{t('mail.to')}:</span>
-            <span>{mail.to_addresses.join(', ')}</span>
+          <div className="flex flex-col sm:flex-row sm:gap-2">
+            <span className="font-medium text-secondary-500 flex-shrink-0">{t('mail.to')}:</span>
+            <span className="break-words">{mail.to_addresses.join(', ')}</span>
           </div>
           {mail.cc_addresses.length > 0 && (
-            <div className="flex gap-2">
-              <span className="font-medium text-secondary-500">{t('mail.cc')}:</span>
-              <span>{mail.cc_addresses.join(', ')}</span>
+            <div className="flex flex-col sm:flex-row sm:gap-2">
+              <span className="font-medium text-secondary-500 flex-shrink-0">{t('mail.cc')}:</span>
+              <span className="break-words">{mail.cc_addresses.join(', ')}</span>
             </div>
           )}
-          <div className="flex gap-2">
-            <span className="font-medium text-secondary-500">{t('mail.date')}:</span>
+          <div className="flex flex-col sm:flex-row sm:gap-2">
+            <span className="font-medium text-secondary-500 flex-shrink-0">{t('mail.date')}:</span>
             <span>{formatFullDate(mail.date)}</span>
           </div>
         </div>
       </div>
 
       {/* Body */}
-      <div className="flex-1 overflow-y-auto px-4 py-4" data-testid="mail-detail-body">
+      <div className="flex-1 overflow-y-auto px-3 py-3 md:px-4 md:py-4" data-testid="mail-detail-body">
         {safeHtml && isSafe ? (
           <div
             className="prose prose-sm max-w-none text-secondary-800"
@@ -190,9 +190,9 @@ export function MailDetail({
 
       {/* Attachments */}
       {mail.attachments && mail.attachments.length > 0 && (
-        <div className="px-4 py-3 border-t border-secondary-200" data-testid="mail-detail-attachments">
+        <div className="px-3 py-2 md:px-4 md:py-3 border-t border-secondary-200" data-testid="mail-detail-attachments">
           <h3 className="text-sm font-semibold text-secondary-700 mb-2">{t('mail.attachments')}</h3>
-          <ul className="space-y-2">
+          <ul className="space-y-1 md:space-y-2">
             {mail.attachments.map((att) => (
               <li key={att.id} className="flex items-center gap-3 p-2 rounded-md hover:bg-secondary-50 min-h-touch">
                 <svg className="w-5 h-5 text-secondary-400 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">

frontend/src/components/mail/MailFolderTree.tsx

@@ -147,7 +147,7 @@ function FolderNode({
         <button
           onClick={() => onSelect(folder.id)}
           className={clsx(
-            'flex-1 flex items-center gap-2 px-2 py-1.5 rounded-md text-sm min-h-touch',
+            'flex-1 flex items-center gap-2 px-2 py-2 md:py-1.5 rounded-md text-sm min-h-touch',
             'motion-safe:transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500',
             isSelected
               ? 'bg-primary-50 text-primary-700 font-medium'
@@ -203,7 +203,7 @@ function AccountNode({
     <div role="treeitem" aria-label={account.email}>
       <button
         onClick={() => setExpanded(!expanded)}
-        className="w-full flex items-center gap-1 px-1 py-1.5 rounded-md text-sm font-semibold text-secondary-900 hover:bg-secondary-50"
+        className="w-full flex items-center gap-1 px-1 py-2 md:py-1.5 rounded-md text-sm font-semibold text-secondary-900 hover:bg-secondary-50 min-h-touch"
         data-testid={`account-${account.id}`}
       >
         <ChevronIcon expanded={expanded} />

frontend/src/components/mail/MailList.tsx

@@ -73,7 +73,7 @@ export function MailList({
             <button
               onClick={() => onSelectMail(mail)}
               className={clsx(
-                'w-full text-left px-4 py-3 hover:bg-secondary-50 min-h-touch motion-safe:transition-colors',
+                'w-full text-left px-3 py-2 md:px-4 md:py-3 hover:bg-secondary-50 min-h-touch motion-safe:transition-colors',
                 'focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500',
                 mail.id === selectedMailId && 'bg-primary-50',
                 !mail.is_seen && 'font-semibold',
@@ -102,7 +102,7 @@ export function MailList({
                     </span>
                     <span className="text-xs text-secondary-400 flex-shrink-0">{formatDate(mail.date)}</span>
                   </div>
-                  <p className={clsx('text-sm truncate mt-0.5', !mail.is_seen ? 'text-secondary-800' : 'text-secondary-600')}>
+                  <p className={clsx('text-xs md:text-sm truncate mt-0.5', !mail.is_seen ? 'text-secondary-800' : 'text-secondary-600')}>
                     {mail.subject || t('mail.noSubject')}
                   </p>
                   {mail.labels && mail.labels.length > 0 && (

frontend/src/components/ui/Modal.tsx

@@ -10,6 +10,7 @@ export interface ModalProps {
   closeOnBackdrop?: boolean;
   closeOnEscape?: boolean;
   showCloseButton?: boolean;
+  fullScreenMobile?: boolean;
 }
 
 const sizeClasses = {
@@ -28,6 +29,7 @@ export function Modal({
   closeOnBackdrop = true,
   closeOnEscape = true,
   showCloseButton = true,
+  fullScreenMobile = false,
 }: ModalProps) {
   const dialogRef = useRef<HTMLDivElement>(null);
   const previouslyFocused = useRef<HTMLElement | null>(null);
@@ -60,7 +62,10 @@ export function Modal({
 
   return (
     <div
-      className="fixed inset-0 z-50 flex items-center justify-center p-4"
+      className={clsx(
+        'fixed inset-0 z-50 flex items-center justify-center',
+        fullScreenMobile ? 'p-0 md:p-4' : 'p-4'
+      )}
       role="dialog"
       aria-modal="true"
       aria-labelledby={title ? 'modal-title' : undefined}
@@ -76,12 +81,14 @@ export function Modal({
         className={clsx(
           'relative bg-white rounded-lg shadow-xl w-full',
           'motion-safe:animate-scale',
+          fullScreenMobile && 'h-full md:h-auto md:max-h-[85vh]',
+          fullScreenMobile ? 'rounded-none md:rounded-lg' : '',
           sizeClasses[size]
         )}
       >
         {title && (
-          <div className="px-6 py-4 border-b border-secondary-200">
-            <h2 id="modal-title" className="text-lg font-semibold text-secondary-900">
+          <div className="px-4 py-3 md:px-6 md:py-4 border-b border-secondary-200">
+            <h2 id="modal-title" className="text-base md:text-lg font-semibold text-secondary-900">
               {title}
             </h2>
           </div>
@@ -89,7 +96,7 @@ export function Modal({
         {showCloseButton && (
           <button
             onClick={onClose}
-            className="absolute top-4 right-4 text-secondary-400 hover:text-secondary-600 min-h-touch min-w-touch flex items-center justify-center rounded-md focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500"
+            className="absolute top-3 right-3 md:top-4 md:right-4 text-secondary-400 hover:text-secondary-600 min-h-touch min-w-touch flex items-center justify-center rounded-md focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500"
             aria-label="Close dialog"
           >
             <svg className="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
@@ -97,7 +104,10 @@ export function Modal({
             </svg>
           </button>
         )}
-        <div className="px-6 py-4 max-h-[70vh] overflow-y-auto">
+        <div className={clsx(
+          'px-4 py-4 md:px-6 overflow-y-auto',
+          fullScreenMobile ? 'max-h-none md:max-h-[70vh] h-full md:h-auto' : 'max-h-[70vh]'
+        )}>
           {children}
         </div>
       </div>

frontend/src/pages/Mail.tsx

@@ -69,6 +69,7 @@ export function MailPage() {
   const [forwardMailState, setForwardMailState] = useState<Mail | null>(null);
   const [downloadingAttachmentId, setDownloadingAttachmentId] = useState<string | null>(null);
   const [searchQuery, setSearchQuery] = useState('');
+  const [activeView, setActiveView] = useState<'folders' | 'list' | 'detail'>('folders');
 
   // Load accounts
   const loadAccounts = useCallback(async () => {
@@ -168,6 +169,7 @@ export function MailPage() {
       setMailsPage(1);
       setSearchQuery('');
       setSelectedMail(null);
+      setActiveView('list');
     },
     [folders],
   );
@@ -175,6 +177,7 @@ export function MailPage() {
   // Handle mail selection
   const handleSelectMail = useCallback(
     async (mail: Mail) => {
+      setActiveView('detail');
       setLoadingMail(true);
       try {
         const detail = await getMail(mail.id);
@@ -435,8 +438,8 @@ export function MailPage() {
         </div>
       )}
 
-      {/* Three-pane layout with resizable panels */}
-      <div className="flex flex-1 overflow-hidden">
+      {/* Desktop: three-pane layout with resizable panels */}
+      <div className="hidden md:flex flex-1 overflow-hidden">
         {/* Folder tree — resizable */}
         <ResizablePanel
           initialWidth={224}
@@ -495,6 +498,82 @@ export function MailPage() {
         </ResizablePanel>
       </div>
 
+      {/* Mobile: single-pane view switching */}
+      <div className="flex md:hidden flex-1 overflow-hidden flex-col">
+        {/* View 1: Folder tree */}
+        {activeView === 'folders' && (
+          <div className="flex-1 overflow-y-auto bg-white" data-testid="mobile-folder-pane">
+            <div className="p-3">
+              <MailFolderTree
+                accounts={accounts}
+                folders={folders}
+                selectedFolderId={selectedFolderId}
+                onSelect={handleSelectFolder}
+                loading={loadingFolders}
+              />
+            </div>
+          </div>
+        )}
+
+        {/* View 2: Mail list */}
+        {activeView === 'list' && (
+          <div className="flex-1 overflow-y-auto bg-white" data-testid="mobile-list-pane">
+            <div className="flex items-center gap-2 px-3 py-2 border-b border-secondary-200 sticky top-0 bg-white z-10">
+              <button
+                onClick={() => setActiveView('folders')}
+                className="inline-flex items-center gap-1 px-2 py-1 rounded text-sm text-secondary-700 hover:bg-secondary-100 min-h-touch"
+                aria-label="Zurück zu Ordnern"
+                data-testid="mobile-back-to-folders"
+              >
+                <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+                  <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 19l-7-7 7-7" />
+                </svg>
+                <span className="text-sm font-medium">Ordner</span>
+              </button>
+            </div>
+            <MailList
+              mails={mails}
+              selectedMailId={selectedMail?.id || null}
+              onSelectMail={handleSelectMail}
+              loading={loadingMails}
+              currentPage={mailsPage}
+              total={mailsTotal}
+              pageSize={PAGE_SIZE}
+              onPageChange={setMailsPage}
+            />
+          </div>
+        )}
+
+        {/* View 3: Mail detail */}
+        {activeView === 'detail' && (
+          <div className="flex-1 overflow-y-auto bg-white" data-testid="mobile-detail-pane">
+            <div className="flex items-center gap-2 px-3 py-2 border-b border-secondary-200 sticky top-0 bg-white z-10">
+              <button
+                onClick={() => setActiveView('list')}
+                className="inline-flex items-center gap-1 px-2 py-1 rounded text-sm text-secondary-700 hover:bg-secondary-100 min-h-touch"
+                aria-label="Zurück zur Liste"
+                data-testid="mobile-back-to-list"
+              >
+                <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+                  <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 19l-7-7 7-7" />
+                </svg>
+                <span className="text-sm font-medium">Zurück</span>
+              </button>
+            </div>
+            <MailDetail
+              mail={selectedMail}
+              loading={loadingMail}
+              onReply={handleReply}
+              onForward={handleForward}
+              onCreateEvent={handleCreateEvent}
+              onToggleFlag={handleToggleFlag}
+              onDownloadAttachment={handleDownloadAttachment}
+              downloadingAttachmentId={downloadingAttachmentId}
+            />
+          </div>
+        )}
+      </div>
+
       <ComposeModal
         open={composeOpen}
         mode={composeMode}