fix(mobile): align Android AGP/Gradle/Kotlin with Flutter 3.41.7 (unbreak Android build)

[AaronGoldsmith]

What

Realigns the Android build's AGP / Gradle / Kotlin versions to the defaults that our pinned Flutter (3.41.7) actually supports:

Tool	Before	After
Android Gradle Plugin	9.2.1	8.11.1
Gradle	9.5.1	8.14
Kotlin	2.3.21	2.2.20

Three lines across settings.gradle.kts and gradle-wrapper.properties.

Why

The Android build has been broken on main since 2026-05-20. Two Renovate PRs bumped the Android toolchain a full major version ahead of what Flutter 3.41.7 supports:

• chore(deps): update gradle to v9 #672 — Gradle → v9
• chore(deps): update plugin com.android.application to v9 #674 — com.android.application (AGP) → v9

Under AGP 9, flutter run fails in a chain of ways:

1. kotlin-android plugin "no longer required since AGP 9.0" (built-in Kotlin).
2. After removing it, the Flutter Gradle plugin throws NullPointerException because AGP 9 "only reads the new DSL."
3. Even past that, third-party plugins that still apply kotlin-android (e.g. app_badge_plus) collide with built-in Kotlin: "Cannot add extension with name 'kotlin', as there is an extension already registered."

The chosen versions aren't arbitrary — they're the exact template defaults this Flutter ships, from flutter_tools/lib/src/android/gradle_utils.dart:

const templateDefaultGradleVersion = '8.14';
const templateAndroidGradlePluginVersion = '8.11.1';
const templateKotlinGradlePluginVersion = '2.2.20';

Testing

• flutter run builds and launches on a physical device (Pixel 3a XL, Android with targetSdk 36) — previously it failed at assembleDebug.
• just mobile-check passes (dart format + flutter analyze: no issues).

Notes / caveats

• This reverts to AGP 8 to unbreak the build against the currently-pinned Flutter (3.41.7). Migrating the app and its plugins to AGP 9 is a larger, separate effort — happy to go that direction instead if maintainers prefer.
• CI does not currently compile the Android target, so it would not detect AGP/Gradle compatibility regressions — which is why chore(deps): update gradle to v9 #672/chore(deps): update plugin com.android.application to v9 #674 sat broken for ~7 weeks. The mobile job (.github/workflows/ci.yml) runs flutter pub get, format check, flutter analyze, and flutter test, but no Android Gradle / assembleDebug build. Suggested follow-up: an Android assembleDebug smoke build in CI, and/or Renovate constraints holding AGP/Gradle back until Flutter supports those majors.