fix(calls): hang up explicitly when answering another call while in one (#195)

Answering from the in-call banner is the only call-to-call switch path; it
started the new embed and let the atom dispose the old one, which just removed
the iframe — our m.call.member in the old room lingered ~17 s (until the
delayed leave expired), so everyone there still saw us in the call.

hangupAndWait moves out of LogoutDialog into plugins/call/hangup.ts and
handleAnswer now hangs up, waits for our membership to clear (bounded 4 s),
disposes the old embed itself (its HangupCall echo would otherwise land after
startCall and clear the NEW embed from the atom — seen in testing), then joins
the new call. Measured headless: old membership gone in 1 s, new call live
with both participants.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/hangup.ts

@@ -0,0 +1,35 @@
+import { MatrixClient } from 'matrix-js-sdk';
+import { CallEmbed } from './CallEmbed';
+
+/**
+ * Ask EC to hang up and wait (bounded) until our own RTC membership has left
+ * the room's session. Used wherever the embed is about to be torn down for a
+ * reason other than the user pressing End — logout (#29) and answering another
+ * call while in one (#195) — because disposing the iframe alone leaves a ghost
+ * `m.call.member` for everyone else until the delayed leave event expires
+ * (~17–30 s measured).
+ */
+export const hangupCallAndWait = async (
+  mx: MatrixClient,
+  embed: CallEmbed,
+  timeoutMs = 4000,
+): Promise<void> => {
+  const myUserId = mx.getUserId();
+  const myDeviceId = mx.getDeviceId();
+  const stillIn = (): boolean =>
+    mx.matrixRTC
+      .getRoomSession(embed.room)
+      .memberships.some((m) => m.sender === myUserId && m.deviceId === myDeviceId);
+  try {
+    await embed.hangup();
+  } catch {
+    // widget already gone — fall through to the wait/timeout
+  }
+  const deadline = Date.now() + timeoutMs;
+  while (stillIn() && Date.now() < deadline) {
+    // eslint-disable-next-line no-await-in-loop
+    await new Promise((r) => {
+      setTimeout(r, 150);
+    });
+  }
+};