feat(voice): mic test with loopback in voice settings

The level meter only measured a stream a call had already opened, so settings
offered no way to check a mic before joining — the panel said as much.

Add startMicTest/stopMicTest on AudioManager: the processed input bus is
routed to the master output through a dedicated gain node, so the loopback can
be disconnected precisely. Settings had deliberately never opened the mic
itself; a mic test cannot honour that, so the test hands the mic back when it
stops.

Releasing needs two independent guards, because the user may join a call
mid-test: AudioManager only stops the exact stream it opened (identity check,
not a flag), and the caller must consent — the UI reads the call state, which
AudioManager cannot, as it does not import stores. Unmounting mid-test tears
the loopback down too.

packages/web/src/audio/AudioManager.ts

@@ -32,6 +32,10 @@ export class AudioManager {
   private rnnoiseReady = false;
   private keepAliveOscillator: OscillatorNode | null = null;
 
+  // Mic test (settings → Voice). See startMicTest().
+  private micTestGain: GainNode | null = null;
+  private micTestStream: MediaStream | null = null;
+
   // Cached `getUserMedia` denial. After a NotAllowedError, subsequent
   // `setInputDevice` calls (e.g. `useLiveKit.syncMic` racing the user's
   // tap on a denial prompt) re-throw the cached error WITHOUT issuing a
@@ -567,6 +571,65 @@ export class AudioManager {
     osc.stop(now + 0.45);
   }
 
+  /**
+   * Mic test: routes the processed input bus to the speakers so the user hears
+   * themselves, outside of any call.
+   *
+   * Settings deliberately never opened the mic on their own — the level meter
+   * only measures a stream that a call had already established. A mic test
+   * cannot honour that, so this is the one path that opens it, and
+   * `stopMicTest` hands it back rather than leaving the mic indicator lit.
+   *
+   * Returns false when the mic could not be opened (denied, unplugged).
+   */
+  async startMicTest(): Promise<boolean> {
+    if (this.micTestGain) return true;
+    const ctx = this.ensureContext();
+    await this.resumeContext();
+
+    const hadStream = this.hasActiveStream();
+    if (!hadStream) {
+      const stream = await this.setInputDevice(this.currentInputDeviceId);
+      if (!stream) return false;
+      // Remember the exact stream we opened, so stopMicTest only ever stops
+      // that one — never a stream something else established meanwhile.
+      this.micTestStream = this.currentStream;
+    }
+
+    this.micTestGain = ctx.createGain();
+    this.inputGain!.connect(this.micTestGain);
+    this.micTestGain.connect(this.getMasterOutput());
+    return true;
+  }
+
+  /**
+   * Tears down the loopback.
+   *
+   * @param allowRelease Whether the mic may be handed back. Only the caller
+   *   knows whether a call has started since the test began — AudioManager
+   *   does not read stores — so releasing needs its consent as well as our own
+   *   record that this test is what opened the stream.
+   */
+  stopMicTest(allowRelease: boolean): void {
+    if (!this.micTestGain) return;
+    try { this.inputGain?.disconnect(this.micTestGain); } catch { /* graph already torn down */ }
+    try { this.micTestGain.disconnect(); } catch { /* already detached */ }
+    this.micTestGain = null;
+
+    if (allowRelease && this.micTestStream && this.currentStream === this.micTestStream) {
+      // Detach listeners before stopping (see `_setInputDeviceImpl`).
+      const tracks = this.currentStream.getTracks();
+      tracks.forEach(t => { t.onended = null; });
+      tracks.forEach(t => t.stop());
+      this.currentStream = null;
+    }
+    this.micTestStream = null;
+  }
+
+  isMicTestActive(): boolean {
+    return this.micTestGain !== null;
+  }
+
   getContext(): AudioContext | null {
     return this.ctx;
   }