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>

pubspec.lock

@@ -288,6 +288,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.1"
+  http:
+    dependency: transitive
+    description:
+      name: http
+      sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.0"
   http_parser:
     dependency: transitive
     description:
@@ -408,6 +416,86 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "9.3.0"
+  open_file:
+    dependency: "direct main"
+    description:
+      name: open_file
+      sha256: b22decdae85b459eac24aeece48f33845c6f16d278a9c63d75c5355345ca236b
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.5.11"
+  open_file_android:
+    dependency: transitive
+    description:
+      name: open_file_android
+      sha256: "58141fcaece2f453a9684509a7275f231ac0e3d6ceb9a5e6de310a7dff9084aa"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.6"
+  open_file_ios:
+    dependency: transitive
+    description:
+      name: open_file_ios
+      sha256: a5acd07ba1f304f807a97acbcc489457e1ad0aadff43c467987dd9eef814098f
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.4"
+  open_file_linux:
+    dependency: transitive
+    description:
+      name: open_file_linux
+      sha256: d189f799eecbb139c97f8bc7d303f9e720954fa4e0fa1b0b7294767e5f2d7550
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.0.5"
+  open_file_mac:
+    dependency: transitive
+    description:
+      name: open_file_mac
+      sha256: cd293f6750de6438ab2390513c99128ade8c974825d4d8128886d1cda8c64d01
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.4"
+  open_file_platform_interface:
+    dependency: transitive
+    description:
+      name: open_file_platform_interface
+      sha256: "101b424ca359632699a7e1213e83d025722ab668b9fd1412338221bf9b0e5757"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.3"
+  open_file_web:
+    dependency: transitive
+    description:
+      name: open_file_web
+      sha256: e3dbc9584856283dcb30aef5720558b90f88036360bd078e494ab80a80130c4f
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.0.4"
+  open_file_windows:
+    dependency: transitive
+    description:
+      name: open_file_windows
+      sha256: d26c31ddf935a94a1a3aa43a23f4fff8a5ff4eea395fe7a8cb819cf55431c875
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.0.3"
+  package_info_plus:
+    dependency: "direct main"
+    description:
+      name: package_info_plus
+      sha256: "16eee997588c60225bda0488b6dcfac69280a6b7a3cf02c741895dd370a02968"
+      url: "https://pub.dev"
+    source: hosted
+    version: "8.3.1"
+  package_info_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: package_info_plus_platform_interface
+      sha256: "202a487f08836a592a6bd4f901ac69b3a8f146af552bbd14407b6b41e1c3f086"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.2.1"
   path:
     dependency: transitive
     description:
@@ -669,6 +757,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.1"
+  win32:
+    dependency: transitive
+    description:
+      name: win32
+      sha256: d7cb55e04cd34096cd3a79b3330245f54cb96a370a1c27adb3c84b917de8b08e
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.15.0"
   xdg_directories:
     dependency: transitive
     description: