feat: improve accessibility across the app

- add useDialogA11y hook: focus trap, Escape handling, and focus restore
  for all custom modal dialogs (Modal, ThemeSelector, Admin modals)
- add skip link, per-route document titles, and focus shift to main on
  SPA route changes
- make notification list items real buttons (keyboard operable) and show
  hover-only controls on keyboard focus
- close sidebar, dropdowns, and context menus with Escape, returning
  focus to their triggers; mark closed mobile sidebar inert
- associate missing form labels (2FA code, DateTimePicker), add missing
  aria-labels on icon-only buttons, aria-expanded/controls on toggles,
  aria-pressed on theme tiles, scope=col on table headers
- announce loading states via role=status
- respect prefers-reduced-motion
- new i18n keys: common.skipToContent/previous/next, admin.userActions

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/components/Sidebar.jsx

@@ -14,6 +14,19 @@ export default function Sidebar({ open, onClose }) {
   const { imprintUrl, privacyUrl } = useBranding();
   const [themeOpen, setThemeOpen] = useState(false);
   const [federationCount, setFederationCount] = useState(0);
+  // On mobile the sidebar is an off-canvas drawer: while closed it must be
+  // inert so its links aren't reachable via Tab or screen readers. On
+  // desktop (lg+) it is always visible and never inert.
+  const [isDesktop, setIsDesktop] = useState(
+    () => window.matchMedia('(min-width: 1024px)').matches
+  );
+
+  useEffect(() => {
+    const mq = window.matchMedia('(min-width: 1024px)');
+    const handleChange = (e) => setIsDesktop(e.matches);
+    mq.addEventListener('change', handleChange);
+    return () => mq.removeEventListener('change', handleChange);
+  }, []);
 
   // Fetch pending federation invitation count
   useEffect(() => {
@@ -50,6 +63,8 @@ export default function Sidebar({ open, onClose }) {
   return (
     <>
       <aside
+        id="app-sidebar"
+        inert={!open && !isDesktop}
         className={`fixed top-0 left-0 z-40 h-full w-64 bg-th-side border-r border-th-border
         transition-transform duration-300 ease-in-out
         ${open ? 'translate-x-0' : '-translate-x-full'} lg:translate-x-0`}
@@ -68,7 +83,7 @@ export default function Sidebar({ open, onClose }) {
           </div>
 
           {/* Navigation */}
-          <nav className="flex-1 px-3 py-4 space-y-1 overflow-y-auto">
+          <nav aria-label={t('nav.navigation')} className="flex-1 px-3 py-4 space-y-1 overflow-y-auto">
             <p className="px-3 mb-2 text-xs font-semibold text-th-text-s uppercase tracking-wider">
               {t('nav.navigation')}
             </p>
@@ -95,6 +110,7 @@ export default function Sidebar({ open, onClose }) {
               </p>
               <button
                 onClick={() => setThemeOpen(!themeOpen)}
+                aria-haspopup="dialog"
                 className="w-full flex items-center gap-3 px-3 py-2.5 rounded-lg text-sm font-medium text-th-text-s hover:text-th-text hover:bg-th-hover transition-all duration-200"
               >
                 <Palette size={18} />