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/models/nextcloud_item.dart

@@ -139,6 +139,7 @@ class NextcloudUserQuota {
   final String userName;
   final String email;
   final String serverVersion;
+  final List<String> groups;
 
   const NextcloudUserQuota({
     required this.usedBytes,
@@ -147,6 +148,7 @@ class NextcloudUserQuota {
     required this.userName,
     required this.email,
     required this.serverVersion,
+    this.groups = const [],
   });
 
   factory NextcloudUserQuota.demo() {
@@ -157,6 +159,7 @@ class NextcloudUserQuota {
       userName: 'Ayush Admin',
       email: 'ayush@cloud.internal',
       serverVersion: 'Nextcloud 29.0.4 Hub 8',
+      groups: ['Family', 'Friends'],
     );
   }
 }