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>

android/app/src/main/res/xml/file_paths.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths>
+    <!-- App-specific external storage: getExternalStorageDirectory() -->
+    <external-files-path name="apk_download" path="." />
+    <!-- Fallback: getTemporaryDirectory() -->
+    <cache-path name="apk_cache" path="." />
+</paths>