feat: mail phase 3 - sorting (date/from/subject, asc/desc)

- Add sort_by and sort_order query params to list_mails endpoint
- Add deleted_at IS NULL filter to exclude soft-deleted mails
- Frontend: sort state in Mail.tsx, sort header bar in MailList
- Sort dropdown (Datum/Absender/Betreff) + asc/desc toggle
- fetchMails API accepts optional sortBy/sortOrder params
- i18n: 6 new sort keys (de/en)

frontend/src/pages/Mail.tsx

@@ -78,6 +78,8 @@ export function MailPage() {
   const [activeView, setActiveView] = useState<'folders' | 'list' | 'detail'>('folders');
   const [selectedMailIds, setSelectedMailIds] = useState<Set<string>>(new Set());
   const [showMoveDropdown, setShowMoveDropdown] = useState(false);
+  const [sortBy, setSortBy] = useState<'date' | 'from' | 'subject'>('date');
+  const [sortOrder, setSortOrder] = useState<'asc' | 'desc'>('desc');
 
   // Load accounts
   const loadAccounts = useCallback(async () => {
@@ -150,7 +152,7 @@ export function MailPage() {
         setMails(result.mails);
         setMailsTotal(result.total);
       } else {
-        const result = await fetchMails(selectedFolderId, mailsPage);
+        const result = await fetchMails(selectedFolderId, mailsPage, sortBy, sortOrder);
         setMails(result.mails);
         setMailsTotal(result.total);
       }
@@ -160,7 +162,7 @@ export function MailPage() {
       setMails([]);
     }
     setLoadingMails(false);
-  }, [selectedFolderId, mailsPage, searchQuery]);
+  }, [selectedFolderId, mailsPage, searchQuery, sortBy, sortOrder]);
 
   useEffect(() => {
     loadMails();
@@ -722,6 +724,13 @@ export function MailPage() {
             selectedMailIds={selectedMailIds}
             onToggleSelect={handleToggleSelect}
             onSelectAll={handleSelectAll}
+            sortBy={sortBy}
+            sortOrder={sortOrder}
+            onSortChange={(by, order) => {
+              setSortBy(by);
+              setSortOrder(order);
+              setMailsPage(1);
+            }}
           />
         </ResizablePanel>
 
@@ -792,6 +801,13 @@ export function MailPage() {
               selectedMailIds={selectedMailIds}
               onToggleSelect={handleToggleSelect}
               onSelectAll={handleSelectAll}
+              sortBy={sortBy}
+              sortOrder={sortOrder}
+              onSortChange={(by, order) => {
+                setSortBy(by);
+                setSortOrder(order);
+                setMailsPage(1);
+              }}
             />
           </div>
         )}