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

src/config/schema/mod.rs

@@ -44,6 +44,7 @@ pub struct AppConfig {
     pub rtsp: RtspConfig,
     pub redfish: RedfishConfig,
     pub watchdog: WatchdogConfig,
+    pub uac: crate::otg::service::UacConfig,
 }
 
 impl AppConfig {
@@ -51,6 +52,7 @@ impl AppConfig {
         if self.hid.backend != HidBackend::Otg {
             self.msd.enabled = false;
             self.otg_network.enabled = false;
+            self.uac.enabled = false;
         }
         self.atx.normalize();
     }