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/services/app_lock_service.dart

@@ -0,0 +1,48 @@
+import 'package:local_auth/local_auth.dart';
+
+/// Thin wrapper around `local_auth`. Deliberately doesn't implement its own
+/// PIN storage/hashing - `authenticate` delegates to whatever the OS
+/// already has configured (fingerprint/face, or the device's own PIN/
+/// pattern/password as a fallback when `biometricOnly` is false), which is
+/// both simpler and more secure than reimplementing credential storage.
+class AppLockService {
+  static final LocalAuthentication _auth = LocalAuthentication();
+
+  /// Whether this device can do *some* form of local auth - biometric
+  /// enrolled, or at minimum a device PIN/pattern/password set up.
+  static Future<bool> isDeviceSupported() async {
+    try {
+      final canCheckBiometrics = await _auth.canCheckBiometrics;
+      if (canCheckBiometrics) return true;
+      return await _auth.isDeviceSupported();
+    } catch (_) {
+      return false;
+    }
+  }
+
+  /// Prompts for biometric or device-credential auth. Returns false (never
+  /// throws) on cancellation, failure, or any platform error, so callers
+  /// can treat every non-true result the same way: stay locked/blocked.
+  static Future<bool> authenticate(String reason) async {
+    try {
+      return await _auth.authenticate(
+        localizedReason: reason,
+        options: const AuthenticationOptions(
+          biometricOnly: false,
+          stickyAuth: true,
+          // Default (true) requires an extra manual confirmation tap after
+          // a *passive* modality like face/iris recognizes you - Android's
+          // own guidance reserves that for confirming risky actions
+          // (a purchase); unlocking the app is exactly the "lower-risk"
+          // case they say to pass false for, and on some devices/Android
+          // versions it also determines whether face is offered as an
+          // option in the prompt at all, not just whether it needs a
+          // second tap to accept.
+          sensitiveTransaction: false,
+        ),
+      );
+    } catch (_) {
+      return false;
+    }
+  }
+}