feat(auth): reset a forgotten password by mail

"Passwort vergessen?" appears on the login screen when SMTP and BASE_URL
are configured. The request always gets the same answer and sends the mail
in the background (at most every 2 minutes per account). The link holds a
random token valid once for one hour; only its SHA-256 is stored. Setting
the new password invalidates all open links and ends every session. The
app removes the token from the address bar and offers a way back for
expired links.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -120,6 +120,17 @@ export const App: React.FC = () => {
     instanceName: 'MindSift',
   });
   const [user, setUser] = useState<User | null>(null);
+  // Link from the password reset mail (?resetToken=…); removed from the address bar right away
+  const [resetToken, setResetToken] = useState<string | null>(() => {
+    try {
+      return new URLSearchParams(window.location.search).get('resetToken');
+    } catch {
+      return null;
+    }
+  });
+  useEffect(() => {
+    if (resetToken) window.history.replaceState(null, '', window.location.pathname);
+  }, [resetToken]);
   const [customThemes, setCustomThemes] = useState<UserTheme[]>([]);
   const allThemes = useMemo(() => [...AVAILABLE_THEMES, ...customThemes.map(customThemeOption)], [customThemes]);
   const [stats, setStats] = useState<DashboardStats>({
@@ -666,11 +677,19 @@ export const App: React.FC = () => {
   }
 
   // Show Setup Wizard or Login Screen if needsSetup or not logged in
-  if (authStatus.needsSetup || !user) {
+  if (authStatus.needsSetup || !user || resetToken) {
     return (
       <SetupAndAuthView
         authStatus={authStatus}
         onAuthenticated={handleAuthenticated}
+        resetToken={resetToken}
+        onResetDone={() => {
+          // The new password ended every session, this one included
+          setResetToken(null);
+          clearToken();
+          setUser(null);
+        }}
+        onResetCancel={() => setResetToken(null)}
       />
     );
   }