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/views/files_view.dart

@@ -381,7 +381,7 @@ class _FilesViewState extends State<FilesView> {
                       ? Icons.visibility_rounded
                       : Icons.visibility_off_rounded,
                   isSelected: provider.showHiddenFiles,
-                  onTap: provider.toggleShowHiddenFiles,
+                  onTap: () => provider.toggleShowHiddenFiles(),
                   tooltip: 'Show hidden files',
                 ),
                 const SizedBox(width: 4),