feat: plugin ribbon toolbar + mail response fixes

- Add pluginToolbarStore for plugin toolbar item registration
- Add PluginToolbar component (slim ribbon bar under TopBar)
- Update AppShell to render PluginToolbar between TopBar and content
- Mail page registers toolbar items (account select, search, compose, sync, settings)
- Remove old mail top bar, use plugin toolbar instead
- Fix mail response: arrays for addresses, add from_name/body_html/sanitized_html/date

frontend/src/components/layout/AppShell.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { Outlet } from 'react-router-dom';
 import { Sidebar } from './Sidebar';
 import { TopBar } from './TopBar';
+import { PluginToolbar } from './PluginToolbar';
 import { ToastContainer } from '@/components/ui/Toast';
 
 export function AppShell() {
@@ -10,6 +11,7 @@ export function AppShell() {
       <Sidebar />
       <div className="flex-1 flex flex-col overflow-hidden">
         <TopBar />
+        <PluginToolbar />
         <main
           className="flex-1 overflow-y-auto focus:outline-none"
           tabIndex={-1}

frontend/src/components/layout/PluginToolbar.tsx

@@ -0,0 +1,86 @@
+import React from 'react';
+import { usePluginToolbarStore, type ToolbarItem } from '@/store/pluginToolbarStore';
+
+function ToolbarButton({ item }: { item: ToolbarItem }) {
+  return (
+    <button
+      onClick={item.onClick}
+      disabled={item.disabled}
+      className={`
+        inline-flex items-center gap-1.5 px-2 py-1 rounded text-xs font-medium
+        transition-colors duration-100
+        ${item.active ? 'bg-primary-100 text-primary-700' : 'text-secondary-600 hover:bg-secondary-100 hover:text-secondary-900'}
+        ${item.disabled ? 'opacity-40 cursor-not-allowed' : 'cursor-pointer'}
+      `}
+      title={item.label}
+    >
+      {item.icon && <span className="w-3.5 h-3.5 flex-shrink-0">{item.icon}</span>}
+      <span className="hidden sm:inline">{item.label}</span>
+    </button>
+  );
+}
+
+function ToolbarSearch({ item }: { item: ToolbarItem }) {
+  return (
+    <input
+      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
+        focus:outline-none focus:border-primary-400 focus:ring-1 focus:ring-primary-300
+        placeholder:text-secondary-400"
+    />
+  );
+}
+
+function ToolbarSelect({ item }: { item: ToolbarItem }) {
+  return (
+    <select
+      value={item.selectValue || ''}
+      onChange={(e) => item.onSelect?.(e.target.value)}
+      className="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"
+    >
+      {item.selectOptions?.map((opt) => (
+        <option key={opt.value} value={opt.value}>{opt.label}</option>
+      ))}
+    </select>
+  );
+}
+
+export function PluginToolbar() {
+  const { items, activePlugin } = usePluginToolbarStore();
+  const visibleItems = items.filter((i) => i.plugin === activePlugin);
+
+  if (visibleItems.length === 0) return null;
+
+  // Group items by group field
+  const groups: Record<string, ToolbarItem[]> = {};
+  for (const item of visibleItems) {
+    const g = item.group || 'default';
+    if (!groups[g]) groups[g] = [];
+    groups[g].push(item);
+  }
+  const groupNames = Object.keys(groups);
+
+  return (
+    <div
+      className="flex items-center gap-1 px-3 py-1 bg-white border-b border-secondary-200 min-h-[36px]"
+      data-testid="plugin-toolbar"
+    >
+      {groupNames.map((groupName, gi) => (
+        <React.Fragment key={groupName}>
+          {gi > 0 && <div className="w-px h-5 bg-secondary-200 mx-1" />}
+          <div className="flex items-center gap-1">
+            {groups[groupName].map((item) => {
+              if (item.type === 'search') return <ToolbarSearch key={item.id} item={item} />;
+              if (item.type === 'select') return <ToolbarSelect key={item.id} item={item} />;
+              return <ToolbarButton key={item.id} item={item} />;
+            })}
+          </div>
+        </React.Fragment>
+      ))}
+    </div>
+  );
+}