fix(android): opt out of AGP 9 built-in Kotlin — KSP incompatible

AGP 9 enables built-in Kotlin by default (`android.builtInKotlin=true`),
which we initially adopted by dropping the `kotlin-android` plugin
alias. But KSP isn't compatible with built-in Kotlin yet — Gradle
errors out with:

  > KSP is not compatible with Android Gradle Plugin's built-in Kotlin.
  > Please disable by adding android.builtInKotlin=false to gradle.properties
  > and apply kotlin("android") plugin

Restored the explicit kotlin-android plugin (root + :app) and added
`android.builtInKotlin=false` to gradle.properties. Revisit when KSP
gains built-in-Kotlin support.

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 removed: AGP 9.0+ auto-enables Kotlin via the
-    // android.builtInKotlin=true default. The serialization + compose-compiler
-    // plugins still need explicit application because they're language-level
-    // (Kotlin compiler) plugins beyond what built-in Kotlin provides.
+    // kotlin-android applied explicitly: AGP 9's built-in Kotlin path is
+    // not compatible with KSP yet. Opted out via
+    // android.builtInKotlin=false in gradle.properties.
+    alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.serialization)
     alias(libs.plugins.compose.compiler)
     alias(libs.plugins.ksp)