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/providers/settings_provider.dart

@@ -4,6 +4,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 
 const _kServerUrl = 'server_url';
 const _kThemeMode = 'theme_mode';
+const _kForgejoRepoUrl = 'forgejo_repo_url';
 
 final sharedPreferencesProvider = Provider<SharedPreferences>((ref) {
   throw UnimplementedError('Override in ProviderScope');
@@ -42,6 +43,23 @@ class ThemeModeNotifier extends StateNotifier<ThemeMode> {
   }
 }
 
+final forgejoRepoUrlProvider =
+    StateNotifierProvider<ForgejoRepoUrlNotifier, String?>((ref) {
+  return ForgejoRepoUrlNotifier(ref.watch(sharedPreferencesProvider));
+});
+
+class ForgejoRepoUrlNotifier extends StateNotifier<String?> {
+  final SharedPreferences _prefs;
+  ForgejoRepoUrlNotifier(this._prefs)
+      : super(_prefs.getString(_kForgejoRepoUrl));
+
+  Future<void> setUrl(String url) async {
+    final clean = url.endsWith('/') ? url.substring(0, url.length - 1) : url;
+    await _prefs.setString(_kForgejoRepoUrl, clean);
+    state = clean;
+  }
+}
+
 final serverUrlProvider = StateNotifierProvider<ServerUrlNotifier, String?>((ref) {
   final prefs = ref.watch(sharedPreferencesProvider);
   return ServerUrlNotifier(prefs);