feat: UAC USB microphone passthrough

- Add UAC1 gadget function (ConfigFS) with optimized endpoint config
- Browser mic capture with Opus encoding via WebCodecs AudioEncoder
- WebSocket audio transport (Opus 64kbps, 100x bandwidth reduction vs raw PCM)
- aplay subprocess for reliable PCM playback to USB gadget
- Settings toggle + ActionBar mic button (hidden when UAC disabled)
- Dynamic PCM device resolution (/proc/asound)
- c_chmask=0 + req_number=4 fixes DWC3 composite isochronous endpoint issue

web/src/api/index.ts

@@ -855,6 +855,7 @@ export {
   msdConfigApi,
   otgConfigApi,
   otgNetworkApi,
+  uacApi,
   atxConfigApi,
   audioConfigApi,
   extensionsApi,