Add app lock, multi-account support, and redesigned account settings

App lock (local_auth) gates the app behind the OS's biometric/device
credential prompt via a new lock screen, layered on top of the existing
Nextcloud session rather than replacing it.

Multi-account support lets users save several Nextcloud logins and switch
between them (AccountStore, per-account browsing prefs, avatar swipe-to-
cycle); logout() now keeps an account's saved credentials/prefs intact so
it can be resumed from a "Continue as..." list, while removeAccount()
stays the only destructive path.

The account settings screen is redesigned around a single profile card
(avatar/name/email, server host, storage quota with groups) with
Refresh Cache/Logout/Remove actions, replacing the old separate "Server
Connection Info" section.

Also fixes activity-tab timestamps: the OCS Activity API reports time as
an ISO 8601 `datetime` string, not the numeric `timestamp` field the code
incorrectly assumed, which made every activity render as Jan 1 1970.

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

lib/main.dart

@@ -6,6 +6,7 @@ import 'package:receive_sharing_intent/receive_sharing_intent.dart';
 import 'models/app_tab.dart';
 import 'providers/server_provider.dart';
 import 'theme/app_theme.dart';
+import 'views/lock_screen_view.dart';
 import 'views/login_view.dart';
 import 'views/search_view.dart';
 import 'views/share_upload_view.dart';
@@ -48,9 +49,11 @@ class NextcloudApp extends StatelessWidget {
           themeMode: provider.themeMode,
           home: provider.isRestoringSession
               ? const _SplashView()
-              : (provider.isLoggedIn
-                    ? const MainShellView()
-                    : const LoginView()),
+              : (!provider.isLoggedIn
+                    ? const LoginView()
+                    : (provider.needsUnlock
+                          ? const LockScreenView()
+                          : const MainShellView())),
         );
       },
     );