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/hooks/useCallJoinLeaveSounds.ts

@@ -0,0 +1,57 @@
+import { useCallback, useEffect, useRef } from 'react';
+import { CallMembership } from 'matrix-js-sdk/lib/matrixrtc/CallMembership';
+import { CallEmbed } from '../plugins/call';
+import { useSetting } from '../state/hooks/settings';
+import { settingsAtom } from '../state/settings';
+import { useMatrixClient } from './useMatrixClient';
+import { useCallMembersChange, useCallSession } from './useCall';
+import { useCallJoined } from './useCallEmbed';
+import { playCallJoinSound, playCallLeaveSound } from '../utils/callSounds';
+
+const membershipKey = (m: CallMembership): string => `${m.sender}|${m.deviceId}`;
+
+/**
+ * Plays a local sound effect when another participant joins or leaves
+ * the call you are in. Style (or off) is configured in Settings → Calls.
+ */
+export function useCallJoinLeaveSounds(embed: CallEmbed): void {
+  const mx = useMatrixClient();
+  const [style] = useSetting(settingsAtom, 'callJoinLeaveSound');
+  const joined = useCallJoined(embed);
+  const session = useCallSession(embed.room);
+
+  const prevKeysRef = useRef<Set<string> | null>(null);
+
+  // Snapshot current members when the session (re)starts so we never play
+  // sounds for participants who were already present.
+  useEffect(() => {
+    prevKeysRef.current = new Set(session.memberships.map(membershipKey));
+  }, [session]);
+
+  useCallMembersChange(
+    session,
+    useCallback(
+      (members: CallMembership[]) => {
+        const next = new Set(members.map(membershipKey));
+        const prev = prevKeysRef.current ?? next;
+        prevKeysRef.current = next;
+
+        if (!joined || style === 'off') return;
+
+        const myPrefix = `${mx.getSafeUserId()}|`;
+        let someoneJoined = false;
+        let someoneLeft = false;
+        next.forEach((key) => {
+          if (!prev.has(key) && !key.startsWith(myPrefix)) someoneJoined = true;
+        });
+        prev.forEach((key) => {
+          if (!next.has(key) && !key.startsWith(myPrefix)) someoneLeft = true;
+        });
+
+        if (someoneJoined) playCallJoinSound(style);
+        if (someoneLeft) playCallLeaveSound(style);
+      },
+      [joined, style, mx],
+    ),
+  );
+}