feat: voice channel user limit (P5-10) + call join/leave sounds (P5-16)

P5-10 Voice Channel User Limit:
- New StateEvent.LotusVoiceLimit (io.lotus.voice_limit) with { max_users }
- RoomVoiceLimit admin control in Room Settings > General > Voice
  (power-level gated via permissions.stateEvent)
- CallPrescreen reads the limit reactively and disables Join with a
  'Channel Full (N/N)' message at capacity; existing members can rejoin

P5-16 Custom Join/Leave Sound Effects:
- useCallJoinLeaveSounds hook wired into CallUtils; detects participant
  join/leave via MatrixRTCSession membership changes (sender|deviceId),
  filters out self, only fires while joined
- Sounds synthesized in-browser with Web Audio (callSounds.ts) - no
  assets bundled; styles Off/Chime/Soft/Retro
- 'Join & Leave Sounds' selector in Settings > Calls (previews on change)

Docs: LOTUS_FEATURES.md, README.md, LOTUS_TODO.md (P5-10/P5-16 marked done)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/utils/callSounds.ts

@@ -0,0 +1,95 @@
+export type CallSoundStyle = 'chime' | 'soft' | 'retro';
+
+let sharedCtx: AudioContext | undefined;
+
+const getAudioContext = (): AudioContext | undefined => {
+  try {
+    if (!sharedCtx || sharedCtx.state === 'closed') sharedCtx = new AudioContext();
+    if (sharedCtx.state === 'suspended') sharedCtx.resume().catch(() => undefined);
+    return sharedCtx;
+  } catch {
+    return undefined;
+  }
+};
+
+type Note = {
+  freq: number;
+  /** Offset from now, in seconds */
+  at: number;
+  /** Duration in seconds */
+  dur: number;
+};
+
+const playNotes = (notes: Note[], type: OscillatorType, peakGain: number): void => {
+  const ctx = getAudioContext();
+  if (!ctx) return;
+  const now = ctx.currentTime;
+  notes.forEach(({ freq, at, dur }) => {
+    const osc = ctx.createOscillator();
+    const gain = ctx.createGain();
+    osc.type = type;
+    osc.frequency.value = freq;
+    const start = now + at;
+    // Short attack/decay envelope to avoid clicks
+    gain.gain.setValueAtTime(0, start);
+    gain.gain.linearRampToValueAtTime(peakGain, start + 0.015);
+    gain.gain.exponentialRampToValueAtTime(0.0001, start + dur);
+    osc.connect(gain);
+    gain.connect(ctx.destination);
+    osc.start(start);
+    osc.stop(start + dur + 0.02);
+  });
+};
+
+const SOUNDS: Record<CallSoundStyle, { join: () => void; leave: () => void }> = {
+  chime: {
+    join: () =>
+      playNotes(
+        [
+          { freq: 587.33, at: 0, dur: 0.12 },
+          { freq: 880, at: 0.1, dur: 0.2 },
+        ],
+        'sine',
+        0.25,
+      ),
+    leave: () =>
+      playNotes(
+        [
+          { freq: 880, at: 0, dur: 0.12 },
+          { freq: 587.33, at: 0.1, dur: 0.2 },
+        ],
+        'sine',
+        0.25,
+      ),
+  },
+  soft: {
+    join: () => playNotes([{ freq: 523.25, at: 0, dur: 0.4 }], 'triangle', 0.18),
+    leave: () => playNotes([{ freq: 392, at: 0, dur: 0.4 }], 'triangle', 0.18),
+  },
+  retro: {
+    join: () =>
+      playNotes(
+        [
+          { freq: 440, at: 0, dur: 0.07 },
+          { freq: 554.37, at: 0.07, dur: 0.07 },
+          { freq: 659.25, at: 0.14, dur: 0.14 },
+        ],
+        'square',
+        0.1,
+      ),
+    leave: () =>
+      playNotes(
+        [
+          { freq: 659.25, at: 0, dur: 0.07 },
+          { freq: 554.37, at: 0.07, dur: 0.07 },
+          { freq: 440, at: 0.14, dur: 0.14 },
+        ],
+        'square',
+        0.1,
+      ),
+  },
+};
+
+export const playCallJoinSound = (style: CallSoundStyle): void => SOUNDS[style]?.join();
+
+export const playCallLeaveSound = (style: CallSoundStyle): void => SOUNDS[style]?.leave();