fix(calls): Reject sends the decline to the ring notification (#313)

handleReject related the MSC4310 decline to the caller's call.member event
(the notification's reference), but MSC4310 and Element Call match a
decline against the rtc.notification event itself, so the caller kept
"Calling…" for the full 90 s after being told "Declined", and declines
from other clients (correctly referencing the notification) didn't stop
Lotus ringing on other devices.

- The incoming call keeps the notification's own event id; Reject relates
  the decline to it.
- The "declined on another device" dismissal and declinedRing accept
  either id (older Lotus builds referenced the membership).
- The missed-call line is skipped for a call we declined.

Verified headless (DM, Bob rings, Reject): the decline used to reference
the membership; it now references the notification. Unit test added.

Fixes #313

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/callRing.test.ts

@@ -67,3 +67,18 @@ test('declinedRing finds our own decline of that ring only (#312)', () => {
   assert.equal(declinedRing(room, '$ring2', '@me:x'), false, "someone else's decline");
   assert.equal(declinedRing(room, '$ring3', '@me:x'), false);
 });
+
+test('declinedRing matches a decline of the notification or of the membership (#313)', () => {
+  const decline = (rel: string) => ({
+    getType: () => 'org.matrix.msc4310.rtc.decline',
+    getSender: () => '@me:x',
+    getRelation: () => ({ event_id: rel }),
+  });
+  const room = stateRoom([], [decline('$notification')]);
+  assert.equal(declinedRing(room, ['$membership', '$notification'], '@me:x'), true);
+  assert.equal(declinedRing(room, ['$membership'], '@me:x'), false);
+  assert.equal(
+    declinedRing(stateRoom([], [decline('$membership')]), ['$membership', undefined], '@me:x'),
+    true,
+  );
+});