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/room-settings/general/General.tsx

@@ -13,6 +13,7 @@ import {
   RoomPublish,
   RoomShareInvite,
   RoomUpgrade,
+  RoomVoiceLimit,
 } from '../../common-settings/general';
 import { useRoomCreators } from '../../../hooks/useRoomCreators';
 import { useRoomPermissions } from '../../../hooks/useRoomPermissions';
@@ -54,6 +55,10 @@ export function General({ requestClose }: GeneralProps) {
                 <RoomEncryption permissions={permissions} />
                 <RoomPublish permissions={permissions} />
               </Box>
+              <Box direction="Column" gap="100">
+                <Text size="L400">Voice</Text>
+                <RoomVoiceLimit permissions={permissions} />
+              </Box>
               <Box direction="Column" gap="100">
                 <Text size="L400">Addresses</Text>
                 <RoomPublishedAddresses permissions={permissions} />