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.lock

@@ -297,22 +297,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.4"
-  ffmpeg_kit_flutter_new_audio:
-    dependency: "direct main"
-    description:
-      name: ffmpeg_kit_flutter_new_audio
-      sha256: "0a698b46cd163c8e9917af75325c84d27871a2a8b2c37de3b40486cd0ab662ae"
-      url: "https://pub.dev"
-    source: hosted
-    version: "2.0.0"
-  ffmpeg_kit_flutter_platform_interface:
-    dependency: transitive
-    description:
-      name: ffmpeg_kit_flutter_platform_interface
-      sha256: addf046ae44e190ad0101b2fde2ad909a3cd08a2a109f6106d2f7048b7abedee
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.2.1"
   file:
     dependency: transitive
     description: