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/state/toast.ts

@@ -27,6 +27,23 @@ export const createDownloadToast = (filename: string, iconSrc?: IconSrc): ToastN
   onClick: () => undefined,
 });
 
+// Build an error/system toast (e.g. a failed command or account action). Mirrors
+// createDownloadToast: folds-free (icon src passed in), empty roomId + no-op
+// onClick so a click only dismisses (never navigates to a room).
+export const createErrorToast = (
+  body: string,
+  iconSrc?: IconSrc,
+  displayName = 'Something went wrong',
+): ToastNotif => ({
+  id: `error-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`,
+  displayName,
+  body,
+  roomName: '',
+  roomId: '',
+  iconSrc,
+  onClick: () => undefined,
+});
+
 const baseAtom = atom<ToastNotif[]>([]);
 
 // Write-only setter used in ClientNonUIFeatures