feat(notfound): add NotFound page with 404 handling and localization

src/App.jsx

@@ -18,6 +18,7 @@ import FederationInbox from './pages/FederationInbox';
 import FederatedRoomDetail from './pages/FederatedRoomDetail';
 import Calendar from './pages/Calendar';
 import OAuthCallback from './pages/OAuthCallback';
+import NotFound from './pages/NotFound';
 
 export default function App() {
   const { user, loading } = useAuth();
@@ -65,8 +66,8 @@ export default function App() {
         <Route path="/federation/rooms/:id" element={<FederatedRoomDetail />} />
       </Route>
 
-      {/* Catch all */}
+      {/* 404 */}
-      <Route path="*" element={<Navigate to="/" />} />
+      <Route path="*" element={<NotFound />} />
     </Routes>
   );
 }

src/i18n/de.json

@@ -412,7 +412,7 @@
     "oauthClientSecret": "Client-Secret",
     "oauthClientSecretHint": "Leer lassen, um das bestehende Secret beizubehalten",
     "oauthDisplayName": "Button-Beschriftung",
-    "oauthDisplayNameHint": "Wird auf der Login-Seite angezeigt, z. B. „Firmen-SSO"",
+    "oauthDisplayNameHint": "Wird auf der Login-Seite angezeigt, z. B. Firmen-SSO",
     "oauthAutoRegister": "Neue Benutzer automatisch registrieren",
     "oauthAutoRegisterHint": "Erstellt automatisch Konten für Benutzer, die sich zum ersten Mal per OAuth anmelden.",
     "oauthSaved": "OAuth-Konfiguration gespeichert",
@@ -588,5 +588,11 @@
       "note": "Der Termin wurde automatisch aus deinem Kalender entfernt.",
       "footer": "Diese Nachricht wurde automatisch von {appName} versendet."
     }
+  },
+  "notFound": {
+    "title": "Seite nicht gefunden",
+    "description": "Die Seite, die du suchst, existiert nicht oder wurde verschoben.",
+    "goBack": "Zurück",
+    "goHome": "Zur Startseite"
   }
 }

src/i18n/en.json

@@ -588,5 +588,11 @@
       "note": "The event has been automatically removed from your calendar.",
       "footer": "This message was sent automatically by {appName}."
     }
+  },
+  "notFound": {
+    "title": "Page not found",
+    "description": "The page you are looking for doesn't exist or has been moved.",
+    "goBack": "Go back",
+    "goHome": "Back to home"
   }
 }

src/pages/NotFound.jsx

@@ -0,0 +1,59 @@
+import { Link } from 'react-router-dom';
+import { useLanguage } from '../contexts/LanguageContext';
+import { Ghost, ArrowLeft, Home } from 'lucide-react';
+
+export default function NotFound() {
+  const { t } = useLanguage();
+
+  return (
+    <div className="min-h-screen flex items-center justify-center p-6 relative overflow-hidden">
+      {/* Animated background */}
+      <div className="absolute inset-0 bg-th-bg">
+        <div className="absolute inset-0 opacity-20">
+          <div className="absolute top-1/3 left-1/3 w-96 h-96 bg-th-accent rounded-full blur-[128px] animate-pulse" />
+          <div className="absolute bottom-1/3 right-1/3 w-72 h-72 bg-purple-500 rounded-full blur-[128px] animate-pulse" style={{ animationDelay: '3s' }} />
+        </div>
+      </div>
+
+      <div className="relative w-full max-w-md text-center">
+        <div className="card p-10 backdrop-blur-xl bg-th-card/80 border border-th-border shadow-2xl rounded-2xl">
+          {/* Ghost icon with subtle animation */}
+          <div className="flex justify-center mb-6">
+            <div className="w-20 h-20 bg-th-accent/10 rounded-full flex items-center justify-center animate-bounce" style={{ animationDuration: '2s' }}>
+              <Ghost size={40} className="text-th-accent" />
+            </div>
+          </div>
+
+          {/* 404 number */}
+          <h1 className="text-7xl font-extrabold text-th-text mb-2 tracking-tight">404</h1>
+
+          <h2 className="text-xl font-semibold text-th-text mb-2">
+            {t('notFound.title')}
+          </h2>
+
+          <p className="text-th-text-s mb-8">
+            {t('notFound.description')}
+          </p>
+
+          {/* Action buttons */}
+          <div className="flex flex-col sm:flex-row items-center justify-center gap-3">
+            <button
+              onClick={() => window.history.back()}
+              className="btn-secondary w-full sm:w-auto px-5 py-2.5 flex items-center justify-center gap-2"
+            >
+              <ArrowLeft size={16} />
+              {t('notFound.goBack')}
+            </button>
+            <Link
+              to="/"
+              className="btn-primary w-full sm:w-auto px-5 py-2.5 flex items-center justify-center gap-2"
+            >
+              <Home size={16} />
+              {t('notFound.goHome')}
+            </Link>
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}