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/lib/keyboard_height_plugin.dart

@@ -0,0 +1,29 @@
+import 'dart:async';
+import 'package:flutter/services.dart';
+
+typedef KeyboardHeightCallback = void Function(double height);
+
+class KeyboardHeightPlugin {
+  static const EventChannel _keyboardHeightEventChannel =
+      EventChannel('keyboardHeightEventChannel');
+
+  StreamSubscription? _keyboardHeightSubscription;
+
+  void onKeyboardHeightChanged(KeyboardHeightCallback callback) {
+    if (_keyboardHeightSubscription != null) {
+      _keyboardHeightSubscription!.cancel();
+    }
+    _keyboardHeightSubscription =
+        _keyboardHeightEventChannel.receiveBroadcastStream().listen(
+      (dynamic height) {
+        callback(height as double);
+      },
+    );
+  }
+
+  void dispose() {
+    if (_keyboardHeightSubscription != null) {
+      _keyboardHeightSubscription!.cancel();
+    }
+  }
+}