fix(call): serve RTC transports to Element Call over MSC4515 — calls work again

Since the upstream v0.25.0 merge, Element Call in widget mode no longer
reads .well-known for its LiveKit transport; it asks the HOST via the
widget API (org.matrix.msc4515.get_rtc_transports, capability
org.matrix.msc4515.rtc_transports). cinny never granted the capability
nor implemented WidgetDriver.getRtcTransports(), so discovery returned
nothing and every join failed with "Call is not supported"
(MISSING_MATRIX_RTC_TRANSPORT) — the [LocalMembership] Multiple
Transport Errors line in the browser console.

- matrix-widget-api 1.17.0 -> 1.18.0 (adds MSC4515; also changes the
  sendDelayedEvent driver signature, adapted — parent delay ids were
  removed from the draft).
- Grant MSC4515RtcTransports in getCallCapabilities.
- CallWidgetDriver.getRtcTransports(): homeserver /rtc/transports
  (MSC4143) first, then the .well-known org.matrix.msc4143.rtc_foci list
  (what matrix.lotusguild.org advertises today). Unit-tested.

Server side needs no change: the well-known already carries the livekit
focus and the JWT service answers.

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "lotus-chat",
   "version": "4.12.3-lotus",
-  "description": "Lotus Chat \u2014 Matrix client for Lotus Guild",
+  "description": "Lotus Chat — Matrix client for Lotus Guild",
   "main": "index.js",
   "type": "module",
   "engines": {
@@ -81,7 +81,7 @@
     "linkify-react": "4.3.3",
     "linkifyjs": "4.3.3",
     "matrix-js-sdk": "41.7.0",
-    "matrix-widget-api": "1.17.0",
+    "matrix-widget-api": "1.18.0",
     "millify": "6.1.0",
     "pdfjs-dist": "6.3.289",
     "prismjs": "1.30.0",