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/widgets/profile_avatar_button.dart

@@ -13,15 +13,18 @@ import '../views/account_view.dart';
 class ProfileAvatarButton extends StatelessWidget {
   const ProfileAvatarButton({super.key});
 
-  void _handleVerticalSwipe(BuildContext context, DragEndDetails details) {
+  Future<void> _handleVerticalSwipe(
+    BuildContext context,
+    DragEndDetails details,
+  ) async {
     final velocity = details.primaryVelocity ?? 0;
     if (velocity.abs() < 250) return;
 
     final provider = context.read<ServerProvider>();
     final target = velocity < 0
-        ? provider.cycleToNextAccount()
-        : provider.cycleToPreviousAccount();
-    if (target == null) return;
+        ? await provider.cycleToNextAccount()
+        : await provider.cycleToPreviousAccount();
+    if (target == null || !context.mounted) return;
 
     HapticFeedback.selectionClick();
     ScaffoldMessenger.of(context).showSnackBar(