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

public/index.html

@@ -343,6 +343,17 @@
                     directly.</span>
                 </div>
               </div>
+              <div class="setting-item">
+                <label class="setting-toggle">
+                  <input type="checkbox" id="setting-force-transcode">
+                  <span class="toggle-slider"></span>
+                </label>
+                <div class="setting-info">
+                  <span class="setting-label">Force Audio Transcode</span>
+                  <span class="setting-hint">Transcode audio to AAC for browser compatibility. Enable if you get video
+                    but no audio (fixes Dolby/AC3/EAC3). Requires ffmpeg-static.</span>
+                </div>
+              </div>
               <div class="setting-item">
                 <div class="setting-info" style="flex: 1;">
                   <span class="setting-label">Stream Output Format</span>