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/otg/uac.rs

@@ -0,0 +1,148 @@
+use std::path::{Path, PathBuf};
+
+use tracing::{debug, info};
+
+use super::configfs::{create_dir, create_symlink, remove_dir, write_file};
+use super::function::GadgetFunction;
+use crate::error::{AppError, Result};
+
+/// USB Audio Class 2.0 (UAC1) gadget function.
+///
+/// Creates a virtual USB microphone that the USB host sees as a standard
+/// USB audio input device. Audio written to the PCM playback device on the
+/// gadget side appears as microphone input on the host.
+#[derive(Debug, Clone)]
+pub struct UacFunction {
+    name: String,
+    sample_rate: u32,
+    channels: u8,
+}
+
+impl UacFunction {
+    /// Create a new UAC1 function instance.
+    ///
+    /// `instance` is a zero-based index to avoid name collisions
+    /// (e.g. `uac2.usb0`).
+    pub fn new(instance: u8, sample_rate: u32, channels: u8) -> Result<Self> {
+        if sample_rate == 0 || sample_rate > 384_000 {
+            return Err(AppError::BadRequest(format!(
+                "invalid UAC sample rate: {sample_rate}"
+            )));
+        }
+        if channels == 0 || channels > 8 {
+            return Err(AppError::BadRequest(format!(
+                "invalid UAC channel count: {channels}"
+            )));
+        }
+        Ok(Self {
+            name: format!("uac1.usb{instance}"),
+            sample_rate,
+            channels,
+        })
+    }
+
+    fn function_path(&self, gadget_path: &Path) -> PathBuf {
+        gadget_path.join("functions").join(&self.name)
+    }
+}
+
+impl GadgetFunction for UacFunction {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn create(&self, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        create_dir(&func_path)?;
+
+        // Playback direction (gadget → host): the controlled machine sees
+        // this as a microphone/line-in input.
+        let chmask: u32 = (1u32 << self.channels) - 1;
+        write_file(&func_path.join("p_chmask"), &chmask.to_string())?;
+        write_file(&func_path.join("p_srate"), &self.sample_rate.to_string())?;
+        write_file(&func_path.join("p_ssize"), "2")?; // 16-bit S16LE
+        // UAC1 does not need p_hs_bint — Windows has native built-in
+        // UAC1 drivers and handles isochronous streaming automatically.
+
+        // Only enable playback direction (gadget → host = mic).
+        // Disabling capture saves one isochronous endpoint.
+        write_file(&func_path.join("c_chmask"), "0")?;
+
+        // req_number=4: explicitly allocate 4 USB requests for the
+        // isochronous endpoint.  Default (0 = auto) may not be enough
+        // for composite gadgets on DWC3.
+        let _ = write_file(&func_path.join("req_number"), "4");
+
+        debug!(
+            "UAC1 function {} created: {}ch {}Hz",
+            &self.name, self.channels, self.sample_rate
+        );
+        Ok(())
+    }
+
+    fn link(&self, config_path: &Path, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        let link_path = config_path.join(&self.name);
+        create_symlink(&func_path, &link_path)?;
+        debug!("UAC1 function {} linked into configuration", &self.name);
+        Ok(())
+    }
+
+    fn unlink(&self, config_path: &Path) -> Result<()> {
+        let link_path = config_path.join(&self.name);
+        if link_path.exists() {
+            std::fs::remove_file(&link_path).map_err(|e| {
+                AppError::Internal(format!(
+                    "Failed to unlink UAC1 function {}: {}",
+                    &self.name, e
+                ))
+            })?;
+            debug!("UAC1 function {} unlinked", &self.name);
+        }
+        Ok(())
+    }
+
+    fn cleanup(&self, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        if func_path.exists() {
+            remove_dir(&func_path).map_err(|e| {
+                AppError::Internal(format!(
+                    "Failed to remove UAC1 function {}: {}",
+                    &self.name, e
+                ))
+            })?;
+            info!("UAC1 function {} removed", &self.name);
+        }
+        Ok(())
+    }
+}
+
+/// Return the ALSA PCM device name that the kernel assigns to a UAC1
+/// gadget after binding. The device appears as a playback-only PCM on
+/// the gadget side.
+pub fn uac_pcm_device() -> String {
+    // The kernel assigns the card name based on the gadget name.
+    // The PCM name is typically "playback" for UAC1.
+    "hw:UAC1Gadget,0".to_string()
+}
+
+/// Resolve the actual PCM device name for a UAC1 playback device
+/// by scanning /proc/asound/ for the gadget audio card.
+pub fn find_uac_pcm_device() -> Option<String> {
+    for entry in std::fs::read_dir("/proc/asound").ok()? {
+        let entry = entry.ok()?;
+        let name = entry.file_name();
+        let name = name.to_str()?;
+        if !name.starts_with("card") {
+            continue;
+        }
+        let card_path = entry.path().join("id");
+        if let Ok(id) = std::fs::read_to_string(&card_path) {
+            if id.trim().starts_with("UAC1Gadget") || id.trim().starts_with("gadget") {
+                let card_num = name.strip_prefix("card")?;
+                return Some(format!("hw:{card_num},0"));
+            }
+        }
+    }
+    None
+}