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>

README.md

@@ -50,6 +50,8 @@ The Lotus Chat logo (`lotus_chat.png`) is a derivative work based on the origina
 - Dark/light mode inside calls matches your Lotus Chat theme
 - Calls are available in DMs and private groups only — no accidental mass rings
 - AFK auto-mute: mic is automatically silenced after a configurable idle timeout (1–30 min); a toast confirms the action
+- Voice channel user limit: admins can cap how many people can be in a room's call; others see "Channel Full" until a spot opens
+- Custom join/leave sound effects when someone enters or leaves your call — choose Chime, Soft, Retro, or off
 
 ### Customization & Appearance