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/features/call/CallView.tsx

@@ -10,6 +10,8 @@ import { useRoomPermissions } from '../../hooks/useRoomPermissions';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { StateEvent } from '../../../types/matrix/room';
 import { useCallMembers, useCallSession } from '../../hooks/useCall';
+import { useStateEvent } from '../../hooks/useStateEvent';
+import { VoiceLimitContent } from '../common-settings/general/RoomVoiceLimit';
 import { CallMemberRenderer } from './CallMemberCard';
 import * as css from './styles.css';
 import { CallControls } from './CallControls';
@@ -74,6 +76,14 @@ function AlreadyInCallMessage() {
   );
 }
 
+function ChannelFullMessage({ current, max }: { current: number; max: number }) {
+  return (
+    <Text style={{ margin: 'auto', color: color.Critical.Main }} size="L400" align="Center">
+      Channel Full ({current}/{max}) — Wait for someone to leave before joining.
+    </Text>
+  );
+}
+
 function CallPrescreen() {
   const mx = useMatrixClient();
   const room = useRoom();
@@ -96,7 +106,14 @@ function CallPrescreen() {
   const callEmbed = useCallEmbed();
   const inOtherCall = callEmbed && callEmbed.roomId !== room.roomId;
 
-  const canJoin = hasPermission && livekitSupported && rtcSupported;
+  // Voice channel user limit (io.lotus.voice_limit). 0 / absent means no limit.
+  const limitEvent = useStateEvent(room, StateEvent.LotusVoiceLimit);
+  const maxUsers = limitEvent?.getContent<VoiceLimitContent>().max_users ?? 0;
+  // A user already counted in the session is rejoining and should not be blocked.
+  const alreadyMember = callMembers.some((m) => m.sender === mx.getSafeUserId());
+  const channelFull = maxUsers > 0 && !alreadyMember && callMembers.length >= maxUsers;
+
+  const canJoin = hasPermission && livekitSupported && rtcSupported && !channelFull;
 
   return (
     <Scroll variant="Surface" hideTrack>
@@ -117,16 +134,17 @@ function CallPrescreen() {
           <CallMemberRenderer members={callMembers} />
           <PrescreenControls canJoin={canJoin} />
           <Box className={css.PrescreenMessage} alignItems="Center">
-            {!inOtherCall &&
-              (hasPermission ? (
-                <JoinMessage
-                  hasParticipant={hasParticipant}
-                  livekitSupported={livekitSupported}
-                  rtcSupported={rtcSupported}
-                />
-              ) : (
-                <NoPermissionMessage />
-              ))}
+            {!inOtherCall && !hasPermission && <NoPermissionMessage />}
+            {!inOtherCall && hasPermission && channelFull && (
+              <ChannelFullMessage current={callMembers.length} max={maxUsers} />
+            )}
+            {!inOtherCall && hasPermission && !channelFull && (
+              <JoinMessage
+                hasParticipant={hasParticipant}
+                livekitSupported={livekitSupported}
+                rtcSupported={rtcSupported}
+              />
+            )}
             {inOtherCall && <AlreadyInCallMessage />}
           </Box>
         </Box>