feat(call): per-participant volume up to 200% via WebAudio gain

HTMLMediaElement.volume caps at 1.0, so boosting a quiet peer past
100% needs an explicit GainNode in the output chain. New
remoteAudioPipelines module owns one AudioContext + GainNode per
remote audio track; attachTrack / detachTrack now create and tear
down the pipeline alongside the LiveKit element.

Once a track is on the WebAudio path its direct output is diverted
(createMediaElementSource semantics), so audio.muted / volume can't
drive output anymore. Deafen, watch-state, manual screen-share mute
and per-user volume are collapsed into one effective-gain formula
that gets recomputed on every state flip — the effect subscribes to
both participantVolumes and screenShareVolumes for live slider drags.

Slider ranges updated to 0–200% across:
- ParticipantVolumeMenu (per-user right-click menu)
- ParticipantsPopover (in-call participant list)
- ScreenShareContextMenu (per-share right-click)

Values above 100% render the percentage in amber as a soft hint that
clipping is possible. Clamp in both volume stores extended to [0, 2]
so persisted values survive.

setAudioOutputDevice now additionally routes via
AudioContext.setSinkId (Chrome 115+) for the WebAudio graph; the
HTMLAudioElement.setSinkId fallback stays for older runtimes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/lib/remoteAudioPipelines.ts

@@ -0,0 +1,151 @@
+// Central registry of Web-Audio pipelines for every remote audio track we're
+// playing. Exists because `HTMLMediaElement.volume` caps at 1.0 — to let a
+// user boost a quiet peer past 100% we need an explicit GainNode in the
+// output chain. Once a track is on the WebAudio path, `audio.muted` / volume
+// no longer drive output (the MediaElementSource diverts samples through the
+// graph), so all gating — deafen, watch-state, manual mute, per-user volume —
+// is collapsed into a single effective-gain value per pipeline.
+//
+// The registry is a plain module-level Map. Attach/detach lifecycle is owned
+// by CallContext's attachTrack/detachTrack helpers; gain recomputation is
+// also triggered from CallContext when any state the formula depends on
+// changes.
+
+export type RemoteTrackSource = 'microphone' | 'screenshare';
+
+export interface RemoteAudioPipeline {
+  /** LiveKit track sid — stable identifier for this track's lifetime. */
+  trackSid: string;
+  participantId: string;
+  trackSource: RemoteTrackSource;
+  audio: HTMLAudioElement;
+  ctx: AudioContext;
+  source: MediaElementAudioSourceNode;
+  gain: GainNode;
+}
+
+const pipelines = new Map<string, RemoteAudioPipeline>();
+
+// Build the WebAudio chain for an already-attached HTMLAudioElement. Returns
+// null when WebAudio isn't available (older browsers) — callers should fall
+// back to `audio.volume` in that case.
+export function createPipeline(
+  audio: HTMLAudioElement,
+  info: { trackSid: string; participantId: string; trackSource: RemoteTrackSource },
+): RemoteAudioPipeline | null {
+  const AudioCtx =
+    window.AudioContext ??
+    (window as unknown as { webkitAudioContext?: typeof AudioContext }).webkitAudioContext;
+  if (!AudioCtx) return null;
+  try {
+    const ctx = new AudioCtx();
+    const source = ctx.createMediaElementSource(audio);
+    const gain = ctx.createGain();
+    // Start silent; the caller (CallContext) applies the correct effective
+    // gain immediately after registering via `setPipelineGain`.
+    gain.gain.value = 0;
+    source.connect(gain);
+    gain.connect(ctx.destination);
+    // createMediaElementSource diverts the element's direct output through
+    // the audio graph. Muting the element is then a double-guard — if the
+    // diversion ever fails (older WebKit), the element stays silent instead
+    // of bypassing the gain chain entirely.
+    audio.muted = true;
+    const pipeline: RemoteAudioPipeline = {
+      trackSid: info.trackSid,
+      participantId: info.participantId,
+      trackSource: info.trackSource,
+      audio,
+      ctx,
+      source,
+      gain,
+    };
+    pipelines.set(info.trackSid, pipeline);
+    return pipeline;
+  } catch (err: unknown) {
+    console.warn('createPipeline failed', err);
+    return null;
+  }
+}
+
+export function destroyPipeline(trackSid: string): void {
+  const p = pipelines.get(trackSid);
+  if (!p) return;
+  pipelines.delete(trackSid);
+  try {
+    p.source.disconnect();
+  } catch {
+    /* ignore */
+  }
+  try {
+    p.gain.disconnect();
+  } catch {
+    /* ignore */
+  }
+  void p.ctx.close().catch(() => {
+    /* ignore */
+  });
+}
+
+export function allPipelines(): IterableIterator<RemoteAudioPipeline> {
+  return pipelines.values();
+}
+
+export function getPipeline(trackSid: string): RemoteAudioPipeline | undefined {
+  return pipelines.get(trackSid);
+}
+
+export function pipelinesFor(
+  participantId: string,
+  source?: RemoteTrackSource,
+): RemoteAudioPipeline[] {
+  const out: RemoteAudioPipeline[] = [];
+  for (const p of pipelines.values()) {
+    if (p.participantId !== participantId) continue;
+    if (source && p.trackSource !== source) continue;
+    out.push(p);
+  }
+  return out;
+}
+
+// Ramp the gain slightly (~20ms) so 0→2x doesn't introduce a click and so
+// rapid slider drags stay smooth. ctx.currentTime is the right anchor —
+// setValueAtTime jumps abruptly.
+export function setPipelineGain(pipeline: RemoteAudioPipeline, value: number): void {
+  const v = clampGain(value);
+  try {
+    pipeline.gain.gain.setTargetAtTime(v, pipeline.ctx.currentTime, 0.02);
+  } catch {
+    // Some contexts in a closed state throw — just direct-assign as a
+    // fallback; worst case the next valid call smooths it out.
+    pipeline.gain.gain.value = v;
+  }
+}
+
+export function clampGain(v: number): number {
+  if (!Number.isFinite(v)) return 0;
+  if (v < 0) return 0;
+  // 2.0 matches Discord's upper bound (200%). Going higher invites clipping
+  // since the source is already peaking at 1.0 for most mics.
+  if (v > 2) return 2;
+  return v;
+}
+
+// Route every pipeline's AudioContext to the given sink. Feature-detects
+// `AudioContext.setSinkId` (Chrome 115+); older runtimes silently keep the
+// default sink, which is the behaviour we had before the WebAudio refactor
+// so this is a strict improvement rather than a regression.
+export async function setAllPipelinesSinkId(deviceId: string): Promise<void> {
+  const maybeId = deviceId.length === 0 ? 'default' : deviceId;
+  for (const p of pipelines.values()) {
+    const ctxAny = p.ctx as unknown as {
+      setSinkId?: (id: string) => Promise<void>;
+    };
+    if (typeof ctxAny.setSinkId !== 'function') continue;
+    try {
+      await ctxAny.setSinkId(maybeId);
+    } catch (err: unknown) {
+      console.warn('AudioContext.setSinkId failed', { trackSid: p.trackSid, err });
+    }
+  }
+}