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/settings/general/General.tsx

@@ -65,6 +65,7 @@ import { useMessageSpacingItems } from '../../../hooks/useMessageSpacing';
 import { useDateFormatItems } from '../../../hooks/useDateFormat';
 import { SequenceCardStyle } from '../styles.css';
 import { useTauriUpdater } from '../../../hooks/useTauriUpdater';
+import { playCallJoinSound } from '../../../utils/callSounds';
 
 type ThemeSelectorProps = {
   themeNames: Record<string, string>;
@@ -1113,6 +1114,15 @@ function Calls() {
   const [deafenKey, setDeafenKey] = useSetting(settingsAtom, 'deafenKey');
   const [afkAutoMute, setAfkAutoMute] = useSetting(settingsAtom, 'afkAutoMute');
   const [afkTimeoutMinutes, setAfkTimeoutMinutes] = useSetting(settingsAtom, 'afkTimeoutMinutes');
+  const [callJoinLeaveSound, setCallJoinLeaveSound] = useSetting(
+    settingsAtom,
+    'callJoinLeaveSound',
+  );
+
+  const handleJoinLeaveSoundChange = (value: 'off' | 'chime' | 'soft' | 'retro') => {
+    setCallJoinLeaveSound(value);
+    if (value !== 'off') playCallJoinSound(value);
+  };
 
   const pttBind = useKeyBind(setPttKey);
   const deafenBind = useKeyBind(setDeafenKey);
@@ -1227,6 +1237,34 @@ function Calls() {
           />
         )}
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Join & Leave Sounds"
+          description="Play a sound when someone joins or leaves a call you are in."
+          after={
+            <select
+              value={callJoinLeaveSound}
+              onChange={(e) =>
+                handleJoinLeaveSoundChange(e.target.value as 'off' | 'chime' | 'soft' | 'retro')
+              }
+              style={{
+                background: 'var(--bg-surface)',
+                color: 'inherit',
+                border: '1px solid var(--border-interactive-normal)',
+                borderRadius: '6px',
+                padding: '4px 8px',
+                fontSize: 'inherit',
+                cursor: 'pointer',
+              }}
+            >
+              <option value="off">Off</option>
+              <option value="chime">Chime</option>
+              <option value="soft">Soft</option>
+              <option value="retro">Retro</option>
+            </select>
+          }
+        />
+      </SequenceCard>
     </Box>
   );
 }