Fix APK build: self-contained Kotlin-DSL Gradle project

[devin-ai-integration]

Summary

CI failed with Your app is using an unsupported Gradle project. Root cause: the repo committed legacy Groovy Gradle files (build.gradle, settings.gradle, wrapper 8.x), while the CI step flutter create . --platforms=android regenerated modern Kotlin-DSL files (AGP 9.x / Gradle 9.x) on top of them — producing an inconsistent, half-migrated Android project on every run.

This PR makes the Android project self-contained and deterministic so flutter build apk --release --no-pub succeeds without flutter create:

• Replace Groovy Gradle files with committed Kotlin DSL equivalents (settings.gradle.kts, build.gradle.kts, app/build.gradle.kts) consistent with Gradle 9.1.0 + AGP, and update gradle-wrapper.properties to match.
• Stop committing the wrapper jar / gradlew (now gitignored); flutter build regenerates them, verified in a clean checkout.
• Drop the fragile flutter create . step from .github/workflows/build_apk.yml; CI now just runs flutter pub get then builds.

- - name: 4. Generar o actualizar el proyecto Android
-   run: |
-     flutter create . --platforms=android --org com.example
-     flutter pub get
+ - name: 4. Obtener dependencias del proyecto
+   run: |
+     flutter pub get

Additional fixes required for the build to compile:

• app/build.gradle.kts: add implementation("com.google.android.material:material:1.12.0") — Theme.EclipseShell extends Theme.MaterialComponents.*, which otherwise fails Android resource linking.
• AndroidManifest.xml: remove the deprecated package attribute (rejected by AGP 8+; namespace comes from Gradle).
• lib/audio/audio_handler.dart: remove a duplicate _defaultScanRoot() declaration and add the missing toggleShuffle() method called from eclipse_shell_app.dart.

Also added a pull_request trigger to the workflow so the APK build is validated on PRs (not only on push to main), guarding against future regressions.

Verification

• Built from a fresh clone of this branch (no wrapper jar / gradlew present, no flutter create):

flutter pub get
flutter build apk --release --no-pub
✓ Built build/app/outputs/flutter-apk/app-release.apk (51.9MB)

• GitHub Actions build check passes on this PR.

Link to Devin session: https://app.devin.ai/sessions/397387253e524159baa36ad526240957
Requested by: @BlazeTrack

[devin-ai-integration]

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