fix: revert echo gate, restore Chrome AEC during screen share

The echo gate architecture bypassed the serialized mic management chain
and introduced race conditions. Chrome's AEC handles echo properly for
both headphone and speaker users without custom intervention.

packages/web/src/audio/AudioManager.ts

@@ -23,7 +23,6 @@ export class AudioManager {
   private voiceEchoCancellation = true;
   private voiceNoiseSuppression = true;
   private voiceAutoGainControl = false;
-  private screenShareActive = false;
   private streamGeneration = 0;
   private inputSwitchChain: Promise<MediaStream | null> = Promise.resolve(null);
   private rnnoiseNode: AudioWorkletNode | null = null;
@@ -203,10 +202,8 @@ export class AudioManager {
         this.currentStream.getTracks().forEach(t => t.stop());
       }
 
-      // When screen sharing with audio, Chrome's AEC uses the getDisplayMedia
-      // audio as a reference signal and ducks the mic — even with headphones.
-      // Force AEC off during screen share to prevent this.
-      const effectiveEchoCancellation = this.screenShareActive ? false : this.voiceEchoCancellation;
+      // Chrome AEC stays on during screen share — headphone users unaffected,
+      // speaker users get proper echo cancellation.
 
       // When RNNoise is active, force browser NS off — running both degrades quality.
       // The user's noiseSuppression preference is preserved in the store for when RNNoise is disabled.
@@ -215,12 +212,12 @@ export class AudioManager {
       const constraints = {
         audio: {
           deviceId: deviceId === 'default' ? undefined : { exact: deviceId },
-          echoCancellation: effectiveEchoCancellation,
+          echoCancellation: this.voiceEchoCancellation,
           noiseSuppression: effectiveNoiseSuppression,
           autoGainControl: this.voiceAutoGainControl,
           // Chromium-specific constraints — belt-and-suspenders to ensure
           // Chrome's internal audio engine respects the standard constraints.
-          googEchoCancellation: effectiveEchoCancellation,
+          googEchoCancellation: this.voiceEchoCancellation,
           googAutoGainControl: this.voiceAutoGainControl,
           googNoiseSuppression: effectiveNoiseSuppression,
           googHighpassFilter: false,
@@ -341,22 +338,6 @@ export class AudioManager {
     }
   }
 
-  /**
-   * When screen sharing with audio is active, Chrome's AEC uses the screen
-   * share audio as a reference signal and aggressively ducks the microphone.
-   * Setting this flag forces echoCancellation OFF regardless of user preference,
-   * severing the software link that causes the ducking.
-   */
-  setScreenShareActive(active: boolean) {
-    if (this.screenShareActive === active) return;
-    this.screenShareActive = active;
-    console.log(`[AudioManager] Screen share active: ${active} — ${active ? 'forcing AEC off' : 'restoring user AEC preference'}`);
-    if (this.currentStream) {
-      this.currentStream.getTracks().forEach(t => t.stop());
-      this.currentStream = null;
-    }
-  }
-
   getStreamGeneration(): number {
     return this.streamGeneration;
   }