fix(errors): surface previously-silent async failures (DP1/DP4/DP6)

Several fire-and-forget promises failed silently:
- RoomInput slash-command exe() rejections reset the editor as if they
  succeeded; now caught and shown via an error toast.
- Favourite / low-priority room-tag toggles (setRoomTag/deleteRoomTag)
  swallowed rejections; now surfaced via the same error toast.
- Call-decline sendEvent(RTCDecline) was uncaught; now logged best-effort
  while the local UI still dismisses.

Adds a shared createErrorToast builder mirroring createDownloadToast.
/kick and /ban route through rateLimitedActions, whose to() helper
swallows non-429 errors, so those two can still resolve on failure — noted
in code; the top-level exe() catch covers everything that does reject.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/CallEmbedProvider.tsx

@@ -544,11 +544,16 @@ function IncomingCallListener({ callEmbed, joined }: IncomingCallListenerProps)
 
   const handleReject = useCallback(
     (room: Room, eventId: string) => {
+      // Best-effort: the local UI dismisses regardless (below), but a failed
+      // decline used to reject uncaught. Log it instead of surfacing — the caller
+      // will time the call out on their side even if our decline never lands.
       mx.sendEvent(room.roomId, EventType.RTCDecline, {
         'm.relates_to': {
           rel_type: RelationType.Reference,
           event_id: eventId,
         },
+      }).catch((err) => {
+        console.error('Failed to send call decline:', err);
       });
       setCallInfo(undefined);
     },