feat(auth): "Angemeldet bleiben"-Checkbox auf Login-Screen

Wenn aktiviert, bekommt der JWT-Token statt der üblichen 7 Tage eine
Lebensdauer von 180 Tagen. Der Token liegt wie bisher in localStorage,
bleibt also bis zum manuellen Löschen / Cookie-Reset gültig.

- backend/routers/auth_router.py: LoginRequest.remember_me, längere
  expires_delta beim Token-Erstellen
- index.html: Checkbox unter dem 2FA-Feld
- api.js: login() reicht remember_me als 4. Parameter durch
- app.js: Wert aus #login-remember lesen und mitschicken
- Version v5 → v6

frontend/js/api.js

@@ -64,8 +64,8 @@ export const api = {
   delete: (path)        => request('DELETE', path),
   upload: (path, form)  => uploadRequest(path, form),
 
-  login: (username, password, totp_code = null) =>
-    request('POST', '/auth/login', { username, password, totp_code }),
+  login: (username, password, totp_code = null, remember_me = false) =>
+    request('POST', '/auth/login', { username, password, totp_code, remember_me }),
 
   setupRequired: () => request('GET', '/auth/setup-required'),
   setup: (data)      => request('POST', '/auth/setup', data),

frontend/js/app.js

@@ -141,11 +141,12 @@ function bindLoginForm() {
     const username = document.getElementById('login-username').value.trim();
     const password = document.getElementById('login-password').value;
     const totpCode = document.getElementById('login-totp')?.value.trim() || null;
+    const remember = document.getElementById('login-remember')?.checked || false;
     const errEl    = document.getElementById('login-error');
     errEl.classList.add('hidden');
 
     try {
-      const res = await api.login(username, password, totpCode);
+      const res = await api.login(username, password, totpCode, remember);
       localStorage.setItem('token', res.access_token);
       localStorage.setItem('user', JSON.stringify(res.user));
       await launchApp();

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v5';
+export const APP_VERSION = 'v6';