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>

linux/flutter/generated_plugin_registrant.cc

@@ -6,6 +6,10 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <open_file_linux/open_file_linux_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
+  g_autoptr(FlPluginRegistrar) open_file_linux_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "OpenFileLinuxPlugin");
+  open_file_linux_plugin_register_with_registrar(open_file_linux_registrar);
 }

linux/flutter/generated_plugins.cmake

@@ -3,6 +3,7 @@
 #
 
 list(APPEND FLUTTER_PLUGIN_LIST
+  open_file_linux
 )
 
 list(APPEND FLUTTER_FFI_PLUGIN_LIST