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/utils/sfx.ts

@@ -0,0 +1,13 @@
+import { useVoiceStore } from '../stores/voiceStore';
+
+/** Base gain applied to every sound effect before the user's SFX slider. */
+export const SFX_BASE_VOLUME = 0.8;
+
+/**
+ * Effective sound-effect gain: the base SFX volume scaled by the user's SFX
+ * slider (0–200, where 100 = unity). Read at play time so volume changes take
+ * effect on the next cue without re-subscribing.
+ */
+export function getSfxVolume(): number {
+  return SFX_BASE_VOLUME * (useVoiceStore.getState().soundEffectVolume / 100);
+}