Pin Android SDK components and aapt2 override for Flutter builds

The Android SDK lives in the read-only Nix store, so the Gradle build
cannot auto-install missing components. Pin everything the Flutter
Android build needs in the dev shell's composeAndroidPackages:
platforms 33-36 (app targets 36; plugins pin 34 and 35), build-tools
35.0.0 (AGP 8.11.1), NDK 28.2.13676358 (flutter.ndkVersion), and
cmake 3.22.1 (plugin native builds).

Also redirect the Android Gradle Plugin to the Nix-patched aapt2 from
the SDK via android.aapt2FromMavenOverride, written to the user-global
~/.gradle/gradle.properties on shell entry (the project's tracked
gradle.properties must not contain a machine-specific store path).
AGP's Maven-downloaded aapt2 cannot run on NixOS.

Ignore the generated frontend/android/build/ directory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/android/.gitignore

@@ -1,5 +1,6 @@
 gradle-wrapper.jar
 /.gradle
+/build/
 /captures/
 /gradlew
 /gradlew.bat