feat: improve SAF file descriptor handling and Android platform compatibility

- Migrate MainActivity from FlutterActivity to FlutterFragmentActivity for SAF picker compatibility
- Add ImpellerAwareFlutterFragment to support Impeller fallback on legacy devices
- Add output_fd support in Go backend for direct file descriptor writes (SAF)
- Add helper functions in output_fd.go for FD-based file operations
- Refactor Tidal/Qobuz/Amazon downloaders to support FD output and skip metadata embedding for SAF (handled by Flutter)
- Add extractQobuzDownloadURLFromBody with unit tests for robust URL parsing
- Add storage mode picker (SAF vs App folder) in download settings for Android
- Fix FFmpeg output path building to avoid same-path conflicts
- Embed metadata to SAF FLAC files via temp file bridge in Flutter
- Upgrade Gradle wrapper to 9.3.1 and add activity-ktx dependency

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-8.14-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.3.1-all.zip