build(android): move to Flutter 3.47 template toolchain (Gradle 9.3.1 / AGP 9.1.0 / Kotlin 2.4.0)

The project pinned Gradle 8.12 / AGP 8.9.1 / Kotlin 2.1.0, all three below the
hard error floors that Flutter 3.47.1 enforces in DependencyVersionChecker.kt
(8.14.0 / 8.11.1 / 2.2.20). Nothing here had rotted; the SDK moved.

Targets Flutter's own template defaults rather than the lowest combo that
clears the floors, because Flutter promotes its warn floor to the error floor
in the very next release -- clearing only the hard floor buys one release of
runway. `flutter-android-preflight` on the host re-derives all of this from
the installed SDK.

Also:
- kotlinOptions{} -> kotlin{compilerOptions{}}: the old DSL is a script
  COMPILATION error under Kotlin >= 2.3, not a warning.
- Java/jvmTarget 11 -> 17.
- Drop android.enableJetifier (obsolete, removed outright in AGP 9). Keep the
  migrator's builtInKotlin=false: builtInKotlin=true makes AGP 9.1.0 supply
  Kotlin 2.2.10, BELOW Flutter's own 2.2.20 floor, so Flutter rejects its own
  recommendation.

This app has no native plugins, so it needed none of the pub-plugin
workarounds gps-speedo did.

Verified: flutter build apk --debug and --release, as both mikkeli and hermes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dKAqFo9Uj3mPZWvDzj3G8

android/gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.3.1-all.zip