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/package.json

@@ -14,6 +14,7 @@
     "clsx": "^2.1.1",
     "lucide-vue-next": "^0.556.0",
     "opus-decoder": "^0.7.11",
+    "opus-media-recorder": "^0.8.0",
     "pinia": "^3.0.4",
     "qrcode.vue": "^3.10.0",
     "reka-ui": "^2.10.1",