build: Android's built-in Kotlin, the three plugins applying KGP themselves patched in third_party/

AGP 9 compiles Kotlin itself (android.builtInKotlin=true in every app),
and warns that applying the Kotlin Gradle Plugin will soon fail.
flutter_web_auth_2 5.1.0, keyboard_height_plugin 0.3.0 and flutter_tts
4.2.5 still apply it unconditionally, so their pub releases are copied
into third_party/ with only their android/build.gradle changed, and
the root overrides them by path (third_party/README.md says when to
drop each). The servers' images copy third_party/, as pub refuses an
override whose path is missing. third_party/ isn't our own code: left
out of the analysis and the format check.

All four apps' APKs build without the warning. A Kotlin daemon's
session file that slipped into Train's android/ goes, and every app's
android/.gitignore keeps .kotlin/ out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

third_party/flutter_web_auth_2/pubspec.yaml

@@ -0,0 +1,62 @@
+name: flutter_web_auth_2
+description: Flutter plugin for authenticating a user with a web service.
+version: 5.1.0
+resolution: workspace
+homepage: https://github.com/ThexXTURBOXx/flutter_web_auth_2
+repository: https://github.com/ThexXTURBOXx/flutter_web_auth_2
+issue_tracker: https://github.com/ThexXTURBOXx/flutter_web_auth_2/issues
+funding:
+  - https://github.com/sponsors/ThexXTURBOXx
+screenshots:
+  - description: "flutter_web_auth_2 in action on Android"
+    path: screen-android.gif
+  - description: "flutter_web_auth_2 in action on iOS"
+    path: screen-ios.gif
+  - description: "flutter_web_auth_2 in action on MacOS"
+    path: screen-macos.gif
+topics:
+  - authentication
+  - network
+  - oauth
+  - web
+
+environment:
+  sdk: ">=3.5.0 <4.0.0"
+  flutter: ">=3.24.0"
+
+dependencies:
+  desktop_webview_window: ^0.3.0
+  flutter:
+    sdk: flutter
+  flutter_web_auth_2_platform_interface: ^5.0.0
+  flutter_web_plugins:
+    sdk: flutter
+  path_provider: ^2.1.2
+  url_launcher: ^6.1.6
+  web: ">=0.5.0 <2.0.0"
+  window_to_front: ^0.0.3
+
+dev_dependencies:
+  flutter_lints: ^6.0.0
+  flutter_test:
+    sdk: flutter
+
+flutter:
+  plugin:
+    platforms:
+      android:
+        package: com.linusu.flutter_web_auth_2
+        pluginClass: FlutterWebAuth2Plugin
+      ios:
+        pluginClass: FlutterWebAuth2Plugin
+      linux:
+        dartPluginClass: FlutterWebAuth2LinowsPlugin
+        fileName: src/linows.dart
+      macos:
+        pluginClass: FlutterWebAuth2Plugin
+      web:
+        pluginClass: FlutterWebAuth2WebPlugin
+        fileName: src/web.dart
+      windows:
+        dartPluginClass: FlutterWebAuth2LinowsPlugin
+        fileName: src/linows.dart