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>

src/app/hooks/useCallEmbed.ts

@@ -16,6 +16,7 @@ import { useCallMembersChange, useCallSession } from './useCall';
 import { CallPreferences } from '../state/callPreferences';
 import { useSetting } from '../state/hooks/settings';
 import { settingsAtom } from '../state/settings';
+import { unlockCallSounds } from '../utils/callSounds';
 
 const CallEmbedContext = createContext<CallEmbed | undefined>(undefined);
 
@@ -84,6 +85,10 @@ export const useCallStart = (dm = false) => {
       if (!container) {
         throw new Error('Failed to start call, No embed container element found!');
       }
+      // startCall is always invoked from a click/tap handler — unlock the Web
+      // Audio context now (within the gesture) so join/leave sounds that fire
+      // later, without any gesture, are audible to everyone in the call.
+      unlockCallSounds();
       const callEmbed = createCallEmbed(
         mx,
         room,