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>

macos/Flutter/GeneratedPluginRegistrant.swift

@@ -6,9 +6,13 @@ import FlutterMacOS
 import Foundation
 
 import flutter_inappwebview_macos
+import open_file_mac
+import package_info_plus
 import shared_preferences_foundation
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
   InAppWebViewFlutterPlugin.register(with: registry.registrar(forPlugin: "InAppWebViewFlutterPlugin"))
+  OpenFilePlugin.register(with: registry.registrar(forPlugin: "OpenFilePlugin"))
+  FPPPackageInfoPlusPlugin.register(with: registry.registrar(forPlugin: "FPPPackageInfoPlusPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
 }