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/state/settings.ts

@@ -131,6 +131,8 @@ export interface Settings {
 
   afkAutoMute: boolean;
   afkTimeoutMinutes: number;
+
+  callJoinLeaveSound: 'off' | 'chime' | 'soft' | 'retro';
 }
 
 const defaultSettings: Settings = {
@@ -204,6 +206,8 @@ const defaultSettings: Settings = {
 
   afkAutoMute: false,
   afkTimeoutMinutes: 10,
+
+  callJoinLeaveSound: 'chime',
 };
 
 export const getSettings = (): Settings => {