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/keyboard_height_plugin/pubspec.yaml

@@ -0,0 +1,72 @@
+name: keyboard_height_plugin
+description: Flutter plugin that emits keyboard height before it shows (ios/android)
+version: 0.3.0
+repository: https://github.com/nschairer/keyboard_height_plugin
+
+environment:
+  sdk: '>=2.19.6 <4.0.0'
+  flutter: ">=2.5.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+  plugin_platform_interface: ^2.0.2
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^2.0.0
+
+# For information on the generic Dart part of this file, see the
+# following page: https://dart.dev/tools/pub/pubspec
+
+# The following section is specific to Flutter packages.
+flutter:
+  # This section identifies this Flutter project as a plugin project.
+  # The 'pluginClass' specifies the class (in Java, Kotlin, Swift, Objective-C, etc.)
+  # which should be registered in the plugin registry. This is required for
+  # using method channels.
+  # The Android 'package' specifies package in which the registered class is.
+  # This is required for using method channels on Android.
+  # The 'ffiPlugin' specifies that native code should be built and bundled.
+  # This is required for using `dart:ffi`.
+  # All these are used by the tooling to maintain consistency when
+  # adding or updating assets for this project.
+  plugin:
+    platforms:
+      android:
+        package: com.nschairer.keyboard_height_plugin
+        pluginClass: KeyboardHeightPlugin
+      ios:
+        pluginClass: KeyboardHeightPlugin
+
+  # To add assets to your plugin package, add an assets section, like this:
+  # assets:
+  #   - images/a_dot_burr.jpeg
+  #   - images/a_dot_ham.jpeg
+  #
+  # For details regarding assets in packages, see
+  # https://flutter.dev/assets-and-images/#from-packages
+  #
+  # An image asset can refer to one or more resolution-specific "variants", see
+  # https://flutter.dev/assets-and-images/#resolution-aware
+
+  # To add custom fonts to your plugin package, add a fonts section here,
+  # in this "flutter" section. Each entry in this list should have a
+  # "family" key with the font family name, and a "fonts" key with a
+  # list giving the asset and other descriptors for the font. For
+  # example:
+  # fonts:
+  #   - family: Schyler
+  #     fonts:
+  #       - asset: fonts/Schyler-Regular.ttf
+  #       - asset: fonts/Schyler-Italic.ttf
+  #         style: italic
+  #   - family: Trajan Pro
+  #     fonts:
+  #       - asset: fonts/TrajanPro.ttf
+  #       - asset: fonts/TrajanPro_Bold.ttf
+  #         weight: 700
+  #
+  # For details regarding fonts in packages, see
+  # https://flutter.dev/custom-fonts/#from-packages