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/android/build.gradle

@@ -0,0 +1,59 @@
+group = 'com.linusu.flutter_web_auth_2'
+version = '1.0-SNAPSHOT'
+
+buildscript {
+    ext.kotlin_version = '2.1.0'
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:8.13.0'
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+apply plugin: 'com.android.library'
+
+android {
+    compileSdk = 36
+
+    if (project.android.hasProperty("namespace")) {
+        namespace = 'com.linusu.flutter_web_auth_2'
+    }
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_11
+        targetCompatibility = JavaVersion.VERSION_11
+    }
+
+
+    sourceSets {
+        main.java.srcDirs += 'src/main/kotlin'
+    }
+
+    defaultConfig {
+        minSdk = 21
+    }
+
+    dependencies {
+        implementation 'androidx.browser:browser:1.9.0'
+        implementation 'androidx.activity:activity-ktx:1.10.1'
+    }
+}
+
+// Halkyon's patch (4 Nov 2026): Kotlin built into the Android Gradle plugin
+// (AGP 9, android.builtInKotlin=true) rather than the Kotlin Gradle plugin
+// applied here, as Flutter's guide for plugin authors has it.
+kotlin {
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_11
+    }
+}