Add self-hosted update system via Forgejo releases

The app now checks for new APK releases on a Forgejo instance and can
download and install them without leaving the app.

Settings:
- New "Update repository URL" field (e.g. https://git.example.com/user/fabled_app)
- "App version" tile with a manual "Check" button and live status display
- Download progress bar and "Install" button appear when update is found

Startup:
- Shell runs a silent background check on first load when a repo URL is set
- A dialog appears automatically if a newer release is found

Provider (update_provider.dart):
- UpdateNotifier: idle → checking → available → downloading → available
- Hits Forgejo /api/v1/repos/{owner}/{repo}/releases/latest
- Parses semver tag, finds .apk asset, downloads with progress via Dio
- Opens the APK with open_file to trigger the system package installer

Android:
- REQUEST_INSTALL_PACKAGES permission
- FileProvider configured for open_file (shares APK URI with installer)
- res/xml/file_paths.xml covers external-files and cache directories

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/app.dart

@@ -9,6 +9,7 @@ import 'providers/auth_provider.dart';
 import 'providers/capture_queue_provider.dart';
 import 'providers/notes_provider.dart';
 import 'providers/settings_provider.dart';
+import 'providers/update_provider.dart';
 import 'providers/tasks_provider.dart';
 import 'screens/auth/login_screen.dart';
 import 'screens/chat/chat_screen.dart';
@@ -127,12 +128,29 @@ final routerProvider = Provider<GoRouter>((ref) {
   );
 });
 
-class _Shell extends ConsumerWidget {
+class _Shell extends ConsumerStatefulWidget {
   final Widget child;
   const _Shell({required this.child});
 
+  @override
+  ConsumerState<_Shell> createState() => _ShellState();
+}
+
+class _ShellState extends ConsumerState<_Shell> {
   static const _tabs = [Routes.notes, Routes.tasks, Routes.conversations];
 
+  @override
+  void initState() {
+    super.initState();
+    // Silent update check on first app load.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      final repoUrl = ref.read(forgejoRepoUrlProvider);
+      if (repoUrl != null && repoUrl.isNotEmpty) {
+        ref.read(updateProvider.notifier).check(repoUrl);
+      }
+    });
+  }
+
   int _tabIndex(String location) {
     for (var i = 0; i < _tabs.length; i++) {
       if (location.startsWith(_tabs[i])) return i;
@@ -140,10 +158,73 @@ class _Shell extends ConsumerWidget {
     return 0;
   }
 
+  void _showUpdateDialog(UpdateState update) {
+    showDialog<void>(
+      context: context,
+      builder: (dialogContext) => Consumer(
+        builder: (context, ref, _) {
+          final state = ref.watch(updateProvider);
+          final isDownloading = state.status == UpdateStatus.downloading;
+          return AlertDialog(
+            title: const Text('Update available'),
+            content: Column(
+              mainAxisSize: MainAxisSize.min,
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                Text('Version ${state.latestVersion} is ready to install.'),
+                if (state.currentVersion != null)
+                  Text(
+                    'Installed: v${state.currentVersion}',
+                    style: Theme.of(context).textTheme.bodySmall,
+                  ),
+                if (isDownloading) ...[
+                  const SizedBox(height: 16),
+                  LinearProgressIndicator(
+                    value: state.downloadProgress > 0
+                        ? state.downloadProgress
+                        : null,
+                  ),
+                  const SizedBox(height: 4),
+                  Text(
+                    'Downloading… '
+                    '${(state.downloadProgress * 100).toStringAsFixed(0)}%',
+                    style: Theme.of(context).textTheme.bodySmall,
+                  ),
+                ],
+              ],
+            ),
+            actions: [
+              TextButton(
+                onPressed: () => Navigator.pop(dialogContext),
+                child: const Text('Later'),
+              ),
+              if (!isDownloading)
+                FilledButton(
+                  onPressed: () => ref
+                      .read(updateProvider.notifier)
+                      .downloadAndInstall(),
+                  child: const Text('Download & Install'),
+                ),
+            ],
+          );
+        },
+      ),
+    );
+  }
+
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  Widget build(BuildContext context) {
+    // Show update dialog once when a new version is detected.
+    ref.listen(updateProvider, (prev, next) {
+      if (next.status == UpdateStatus.available &&
+          prev?.status != UpdateStatus.available) {
+        WidgetsBinding.instance
+            .addPostFrameCallback((_) => _showUpdateDialog(next));
+      }
+    });
     final location = GoRouterState.of(context).matchedLocation;
     final index = _tabIndex(location);
+    final child = widget.child;
     // Use NavigationRail whenever the screen is wide enough — covers both
     // phone landscape and tablets in either orientation (600 dp breakpoint).
     final isWide = MediaQuery.of(context).size.width >= 600;