chore(android): bump toolchain to Gradle 9.1 + AGP 9.0.1 + Kotlin 2.2.21 for JDK 25

Original 8.x toolchain choked on the ci-android image's JDK 25 with an
opaque "25.0.3" error in `ktlintCheck`; Gradle 8.10's JDK compat matrix
caps at 23. Modern chain:

  - Gradle 9.1.0 (first to support JDK 25)
  - AGP 9.0.1 (requires Gradle 9.1+, requires Kotlin 2.2.10+)
  - Kotlin 2.2.21 / KSP 2.2.21-2.0.5 (latest 2.2.x line)
  - Compose BOM 2026.05.01 (current; pulls ui-text-google-fonts at the
    BOM-managed version, so the explicit pin was dropped)

AGP 9.0 breaking changes that affected us:
  - `kotlin-android` plugin no longer needed — AGP 9 auto-enables via
    `android.builtInKotlin=true` default. Removed alias from both the
    root build.gradle.kts and :app/build.gradle.kts.
  - `applicationVariants` API removed; we don't use it.
  - Other defaults flipped (useAndroidx, uniquePackageNames, etc.) but
    we already set them explicitly or weren't relying on the old defaults.

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

android/app/build.gradle.kts

@@ -1,6 +1,9 @@
 plugins {
     alias(libs.plugins.android.application)
-    alias(libs.plugins.kotlin.android)
+    // 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.
     alias(libs.plugins.kotlin.serialization)
     alias(libs.plugins.compose.compiler)
     alias(libs.plugins.ksp)