feat: use custom FFmpeg AAR for Android, reduce APK size

- Replace ffmpeg_kit_flutter plugin with custom AAR (arm64 + arm7a only)
- Add MethodChannel bridge for FFmpeg in MainActivity
- Create separate pubspec_ios.yaml for iOS builds with ffmpeg_kit plugin
- Update GitHub workflow to swap pubspec for iOS builds
- Reduces Android APK size by ~50MB

pubspec.yaml

@@ -1,7 +1,7 @@
 name: spotiflac_android
 description: Download Spotify tracks in FLAC from Tidal, Qobuz & Amazon Music
 publish_to: 'none'
-version: 2.0.6+36
+version: 2.0.7+37
 
 environment:
   sdk: ^3.10.0
@@ -50,8 +50,8 @@ dependencies:
   receive_sharing_intent: ^1.8.1
   logger: ^2.5.0
   
-  # FFmpeg for audio conversion (audio-only version - much smaller)
-  ffmpeg_kit_flutter_new_audio: ^2.0.0
+  # FFmpeg - using local custom AAR (arm64-v8a + armeabi-v7a only)
+  # ffmpeg_kit_flutter_new_audio: ^2.0.0  # Replaced with local AAR
   open_filex: ^4.7.0
   
   # Notifications