fix: make call join/leave sounds audible to all participants + server-side hard voice limit docs

Sounds (P5-16): browsers block the Web Audio context until a user gesture
starts it, so join/leave sounds — which fire later with no gesture — were
silent. unlockCallSounds() now primes/resumes the shared AudioContext inside
the Join click (centralized in useCallStart so every join path is covered),
making the per-client sounds reliably audible to everyone in the call.

Voice limit (P5-10): the limit is now a hard, cross-client server-side cap
enforced by the voice-limit-guard sidecar (matrix repo) that fronts
lk-jwt-service and refuses LiveKit tokens when a room is full. Updated
LOTUS_FEATURES.md / README.md / LOTUS_TODO.md to reflect that the client
'Channel Full' check is UX only and the server is authoritative.

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

README.md

@@ -50,7 +50,7 @@ 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
+- Voice channel user limit: admins can cap how many people can be in a room's call — enforced server-side for every Matrix client (not just Lotus Chat); 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