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/ios/Classes/KeyboardHeightPlugin.swift

@@ -0,0 +1,47 @@
+import Flutter
+import UIKit
+
+public class KeyboardHeightPlugin: NSObject, FlutterPlugin, FlutterStreamHandler {
+  private var eventSink: FlutterEventSink?
+  private var eventChannel: FlutterEventChannel?
+
+  public static func register(with registrar: FlutterPluginRegistrar) {
+    let instance = KeyboardHeightPlugin()
+    instance.eventChannel = FlutterEventChannel(name: "keyboardHeightEventChannel", binaryMessenger: registrar.messenger())
+    instance.eventChannel?.setStreamHandler(instance)
+  }
+
+  public func onListen(withArguments arguments: Any?, eventSink events: @escaping FlutterEventSink) -> FlutterError? {
+    eventSink = events
+    registerKeyboardObservers()
+    return nil
+  }
+
+  public func onCancel(withArguments arguments: Any?) -> FlutterError? {
+    eventSink = nil
+    unregisterKeyboardObservers()
+    return nil
+  }
+
+  private func registerKeyboardObservers() {
+    NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShow(notification:)), name: UIResponder.keyboardWillShowNotification, object: nil)
+    NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide(notification:)), name: UIResponder.keyboardWillHideNotification, object: nil)
+  }
+
+  private func unregisterKeyboardObservers() {
+    NotificationCenter.default.removeObserver(self, name: UIResponder.keyboardWillShowNotification, object: nil)
+    NotificationCenter.default.removeObserver(self, name: UIResponder.keyboardWillHideNotification, object: nil)
+  }
+
+  @objc private func keyboardWillShow(notification: NSNotification) {
+    if let userInfo = notification.userInfo,
+       let keyboardFrame = userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect {
+      eventSink?(NSNumber(value: keyboardFrame.height))
+    }
+  }
+
+  @objc private func keyboardWillHide(notification: NSNotification) {
+    eventSink?(NSNumber(value: 0.0))
+  }
+}
+