chore(android): bump Kotlin 2.2 -> 2.3 + KSP 2.0 -> 2.3 for AGP 9 built-in Kotlin

The previous attempt opted out of AGP 9's built-in Kotlin (via
android.builtInKotlin=false + explicit kotlin-android plugin) because
the message from Gradle suggested it. But Kotlin 2.2.21's
kotlin-android plugin can't cast AGP 9's new ApplicationExtension to
the removed BaseExtension:

  class ApplicationExtensionImpl$AgpDecorated_Decorated
  cannot be cast to class com.android.build.gradle.BaseExtension

That suggestion is for projects with an older Kotlin toolchain. The
real fix:

  - Kotlin 2.3.21 (latest stable; first line where kotlin-android also
    supports AGP 9, but more importantly the built-in path works)
  - KSP 2.3.8 — KSP PR #2674 (merged Oct 2025) added AGP 9 built-in
    Kotlin support. KSP 1.x and pre-2.3 don't work with built-in Kotlin.
  - Re-drop the kotlin-android plugin from both build.gradle.kts files;
    AGP 9 enables built-in Kotlin by default and KSP 2.3 cooperates.
  - Remove android.builtInKotlin=false from gradle.properties.

compose-compiler plugin tracks the Kotlin version via version.ref, so
no separate bump there.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -1,9 +1,9 @@
 plugins {
     alias(libs.plugins.android.application)
-    // kotlin-android applied explicitly: AGP 9's built-in Kotlin path is
+    // kotlin-android NOT applied: AGP 9 enables built-in Kotlin by default,
-    // not compatible with KSP yet. Opted out via
+    // and KSP 2.3.x supports it (PR #2674, merged Oct 2025). serialization
-    // android.builtInKotlin=false in gradle.properties.
+    // + compose-compiler are language-level Kotlin compiler plugins and
-    alias(libs.plugins.kotlin.android)
+    // still need explicit application.
     alias(libs.plugins.kotlin.serialization)
     alias(libs.plugins.compose.compiler)
     alias(libs.plugins.ksp)

android/build.gradle.kts

@@ -1,9 +1,9 @@
 plugins {
     alias(libs.plugins.android.application) apply false
-    // kotlin-android needed despite AGP 9's built-in Kotlin: KSP isn't
+    // kotlin-android NOT registered: AGP 9 built-in Kotlin + KSP 2.3.x
-    // compatible with the builtInKotlin path yet, so we opt out via
+    // covers the Kotlin compilation path. Language-level Kotlin compiler
-    // android.builtInKotlin=false in gradle.properties.
+    // plugins (serialization, compose-compiler) are still applied
-    alias(libs.plugins.kotlin.android) apply false
+    // explicitly per module.
     alias(libs.plugins.kotlin.serialization) apply false
     alias(libs.plugins.ksp) apply false
     alias(libs.plugins.hilt) apply false

android/gradle.properties

@@ -4,10 +4,4 @@ org.gradle.caching=true
 org.gradle.configuration-cache=true
 android.useAndroidX=true
 android.nonTransitiveRClass=true
-
-# KSP isn't compatible with AGP 9's built-in Kotlin support (the
-# `android.builtInKotlin=true` default). Opt out so we can apply the
-# explicit kotlin-android plugin and KSP together. Revisit when KSP
-# adds built-in-Kotlin support.
-android.builtInKotlin=false
 kotlin.code.style=official

android/gradle/libs.versions.toml

@@ -1,11 +1,15 @@
 [versions]
-# Pinned for JDK 25 compat: Gradle 9.1.0 supports JDK 25, AGP 9.0.1
+# Pinned for JDK 25 + AGP 9 + KSP-built-in-Kotlin compat:
-# requires Gradle 9.1.0+, AGP 9.0 requires Kotlin Gradle Plugin 2.2.10+,
+#   - Gradle 9.1.0 supports JDK 25 (see gradle-wrapper.properties)
-# KSP follows Kotlin version. See gradle-wrapper.properties for the
+#   - AGP 9.0.1 requires Gradle 9.1.0+
-# Gradle version itself.
+#   - Kotlin 2.3.x: AGP 9's built-in Kotlin path; the older kotlin-android
+#     plugin can't cast AGP 9's ApplicationExtension to the removed
+#     BaseExtension, and KSP 1.x isn't compatible with built-in Kotlin
+#   - KSP 2.3.x (PR #2674, merged Oct 2025): adds AGP 9 built-in Kotlin
+#     support, so we don't need the kotlin-android plugin at all
 agp = "9.0.1"
-kotlin = "2.2.21"
+kotlin = "2.3.21"
-ksp = "2.2.21-2.0.5"
+ksp = "2.3.8"
 hilt = "2.59.2"
 hilt-androidx = "1.2.0"
 compose-bom = "2026.05.01"