fix: auto-disable AEC during screen share to prevent Chrome voice ducking

Chrome's AEC uses getDisplayMedia audio as a reference signal and
aggressively ducks the microphone even when headphones are used.
This adds a screenShareActive flag to AudioManager that forces
echoCancellation off during screen share, plus Chromium-specific
goog* constraints as belt-and-suspenders.

packages/web/src/components/modals/UserSettings.tsx

@@ -127,13 +127,13 @@ export function UserSettingsModal() {
             Voice Processing
           </h3>
           <p className="text-xs text-discord-text-muted mb-3">
-            Disable Auto Gain Control when streaming to prevent your browser from ducking your microphone.
+            Echo Cancellation is automatically disabled while you screen share to prevent Chrome from ducking your mic volume.
           </p>
 
           <div className="flex items-center justify-between py-2">
             <div>
               <div className="text-sm text-discord-text-primary">Echo Cancellation</div>
-              <div className="text-xs text-discord-text-muted">Removes echo when using speakers</div>
+              <div className="text-xs text-discord-text-muted">Removes echo when using speakers (auto-disabled during screen share)</div>
             </div>
             <button
               onClick={() => setEchoCancellation(!echoCancellation)}

packages/web/src/components/voice/DmCallView.tsx

@@ -4,6 +4,7 @@ import { useServerStore } from '../../stores/serverStore';
 import { useAuthStore } from '../../stores/authStore';
 import { getActiveRoom } from '../../hooks/useLiveKit';
 import { wsSend } from '../../hooks/useWebSocket';
+import { AudioManager } from '../../audio/AudioManager';
 import { VideoPresets, VideoPreset } from 'livekit-client';
 
 const QUALITY_MAP: Record<string, any> = {
@@ -94,9 +95,11 @@ export function DmCallView() {
     if (!room) return;
     try {
       if (!isScreenSharing) {
+        AudioManager.getInstance().setScreenShareActive(true);
         await room.localParticipant.setScreenShareEnabled(true, { audio: true });
       } else {
         await room.localParticipant.setScreenShareEnabled(false);
+        AudioManager.getInstance().setScreenShareActive(false);
       }
       toggleScreenShare();
     } catch (err) {

packages/web/src/components/voice/StreamTile.tsx

@@ -2,6 +2,7 @@ import React, { useRef, useEffect, useState, useCallback } from 'react';
 import { Avatar } from '../ui/Avatar';
 import { useVoiceStore } from '../../stores/voiceStore';
 import { getActiveRoom, setStreamSubscription } from '../../hooks/useLiveKit';
+import { AudioManager } from '../../audio/AudioManager';
 import { VideoQualityPopover } from './VideoQualityPopover';
 import type { StreamTile as StreamTileType } from '../../hooks/useLiveKit';
 
@@ -107,6 +108,7 @@ export function StreamTile({ tile, large }: StreamTileProps) {
     const room = getActiveRoom();
     if (room) {
       await room.localParticipant.setScreenShareEnabled(false);
+      AudioManager.getInstance().setScreenShareActive(false);
       useVoiceStore.getState().toggleScreenShare();
     }
   }, []);

packages/web/src/components/voice/VoiceControlBar.tsx

@@ -3,6 +3,7 @@ import { useVoiceStore } from '../../stores/voiceStore';
 import { useUIStore } from '../../stores/uiStore';
 import { getActiveRoom } from '../../hooks/useLiveKit';
 import { wsSend } from '../../hooks/useWebSocket';
+import { AudioManager } from '../../audio/AudioManager';
 import { VideoQualityPopover } from './VideoQualityPopover';
 import { VideoPresets, VideoPreset } from 'livekit-client';
 
@@ -97,9 +98,11 @@ export function VoiceControlBar() {
     if (!room) return;
     try {
       if (!isScreenSharing) {
+        AudioManager.getInstance().setScreenShareActive(true);
         await room.localParticipant.setScreenShareEnabled(true, { audio: true });
       } else {
         await room.localParticipant.setScreenShareEnabled(false);
+        AudioManager.getInstance().setScreenShareActive(false);
       }
       toggleScreenShare();
     } catch (err) {

packages/web/src/components/voice/VoiceControls.tsx

@@ -3,6 +3,7 @@ import { useVoiceStore } from '../../stores/voiceStore';
 import { useServerStore } from '../../stores/serverStore';
 import { getActiveRoom } from '../../hooks/useLiveKit';
 import { wsSend } from '../../hooks/useWebSocket';
+import { AudioManager } from '../../audio/AudioManager';
 import { VideoQualityPopover } from './VideoQualityPopover';
 
 /**
@@ -43,9 +44,11 @@ export function VoiceControls() {
     if (!room) return;
     try {
       if (!isScreenSharing) {
+        AudioManager.getInstance().setScreenShareActive(true);
         await room.localParticipant.setScreenShareEnabled(true, { audio: true });
       } else {
         await room.localParticipant.setScreenShareEnabled(false);
+        AudioManager.getInstance().setScreenShareActive(false);
       }
       toggleScreenShare();
     } catch (err) {