feat(sounds): self-made Ogg cues + watch/mute/pop fixes

Replace ripped Discord audio with self-authored Ogg Vorbis files (avoids
copyright on open-source launch) and fix three cue bugs:

- Anti-pop envelope: playSound now applies a 10ms fade-in (+ tail fade-out
  for one-shots) so buffers no longer start at a non-zero amplitude. Kills
  the pop on the looping call cues. Mirrors playTestTone.
- Deafen no longer plays mute+deafen at once: toggleDeafen flips isMuted and
  isDeafened atomically, so SoundController saw both transitions. New tested
  pure helper selectVoiceStateSound() suppresses the side-effect mute cue.
- Viewer-side watch feedback: stream_user_joined/left now play locally on the
  watcher's own machine for explicit Watch/Stop actions, via
  handleViewerWatchToggle. Direct local playback (not a watchingStreams diff)
  keeps auto-teardown silent and works identically on Safari and Electron.

Docs: docs/systems/sounds.md updated (Ogg, dual-audience cue rows, mute/deafen
selection, viewer feedback, anti-pop envelope).

packages/web/src/audio/AudioManager.ts

@@ -169,7 +169,7 @@ export class AudioManager {
     if (!this.ctx) this.initContext();
     
     try {
-      const response = await fetch(`/sounds/${name}.mp3`);
+      const response = await fetch(`/sounds/${name}.ogg`);
       if (!response.ok) throw new Error(`Failed to load sound: ${name}`);
       const arrayBuffer = await response.arrayBuffer();
       const audioBuffer = await this.ctx!.decodeAudioData(arrayBuffer);
@@ -191,12 +191,34 @@ export class AudioManager {
     source.loop = options.loop || false;
 
     const gainNode = this.ctx.createGain();
-    gainNode.gain.value = options.volume ?? 0.8;
+    const targetVolume = options.volume ?? 0.8;
+
+    // Short attack/release envelope to avoid the click/pop that occurs when a
+    // buffer starts (or ends) at a non-zero sample amplitude. Without this the
+    // waveform's first sample jumps from silence instantly, producing an
+    // audible pop — most noticeable on the looping call cues. Mirrors the
+    // envelope used by playTestTone().
+    const now = this.ctx.currentTime;
+    const attack = 0.01; // 10ms fade-in — kills the start pop, imperceptible
+    gainNode.gain.setValueAtTime(0, now);
+    gainNode.gain.linearRampToValueAtTime(targetVolume, now + attack);
+
+    // Tail fade-out for one-shots so they don't pop at the natural end of the
+    // buffer. Looping cues are stopped explicitly by the caller, so they ramp
+    // in once and hold; no tail ramp is scheduled for them.
+    if (!source.loop) {
+      const release = 0.01; // 10ms fade-out
+      const dur = buffer.duration;
+      if (dur > attack + release) {
+        gainNode.gain.setValueAtTime(targetVolume, now + dur - release);
+        gainNode.gain.linearRampToValueAtTime(0, now + dur);
+      }
+    }
 
     source.connect(gainNode);
     gainNode.connect(this.masterBoost!);
 
-    source.start(0);
+    source.start(now);
     return source;
   }