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/web/handlers/config/uac.rs

@@ -0,0 +1,36 @@
+use std::sync::Arc;
+
+use axum::{extract::State, Json};
+
+use crate::error::Result;
+use crate::otg::service::UacConfig;
+use crate::state::AppState;
+
+use super::apply::try_apply_lock;
+
+pub async fn get_uac_config(State(state): State<Arc<AppState>>) -> Json<UacConfig> {
+    Json(state.config.get().uac.clone())
+}
+
+pub async fn update_uac_config(
+    State(state): State<Arc<AppState>>,
+    Json(req): Json<UacConfig>,
+) -> Result<Json<UacConfig>> {
+    req.validate()?;
+    let _guard = try_apply_lock(&state.config_apply_locks.otg, "uac")?;
+
+    let old_config = (*state.config.get()).clone();
+    let mut new_config = old_config.clone();
+    new_config.uac = req;
+
+    state
+        .config
+        .update(|config| {
+            config.uac = new_config.uac.clone();
+        })
+        .await?;
+
+    super::apply::apply_usb_config(&state, &old_config, &new_config).await?;
+
+    Ok(Json(state.config.get().uac.clone()))
+}