feat: Add FFmpeg audio transcoding for Dolby/AC3/EAC3 compatibility

- Add /api/transcode endpoint for audio-only transcoding
- Video passthrough (no re-encoding) for fast performance
- Audio transcode to AAC for browser compatibility
- Add 'Force Audio Transcode' setting toggle in UI
- Add ffmpeg-static as optional dependency
- Handle Electron asar.unpacked path for packaged apps

package.json

@@ -12,6 +12,9 @@
         "express": "^4.18.2",
         "xml2js": "^0.6.2"
     },
+    "optionalDependencies": {
+        "ffmpeg-static": "^5.2.0"
+    },
     "engines": {
         "node": ">=18.0.0"
     }