feat(calls): configurable ringtone volume in Settings (Bug #4 partial)

Adds 'Ringtone Volume' slider (0–100, default 70%) to Settings → Calls.
The IncomingCall audio element reads the setting and applies it as
audioElement.volume before playing, replacing the implicit browser
default of 1.0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/components/CallEmbedProvider.tsx

@@ -104,6 +104,7 @@ function IncomingCall({ dm, info, onIgnore, onAnswer, onReject }: IncomingCallPr
   const { room } = info;
 
   const audioRef = useRef<HTMLAudioElement>(null);
+  const [ringtoneVolume] = useSetting(settingsAtom, 'ringtoneVolume');
 
   const roomName = useRoomName(room);
   const roomAvatar = useRoomAvatar(room, dm);
@@ -126,8 +127,10 @@ function IncomingCall({ dm, info, onIgnore, onAnswer, onReject }: IncomingCallPr
 
   const playSound = useCallback(() => {
     const audioElement = audioRef.current;
-    audioElement?.play().catch(() => undefined);
-  }, []);
+    if (!audioElement) return;
+    audioElement.volume = Math.max(0, Math.min(1, ringtoneVolume / 100));
+    audioElement.play().catch(() => undefined);
+  }, [ringtoneVolume]);
 
   useEffect(() => {
     const audioEl = audioRef.current;