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/features/room/RoomInput.tsx

@@ -136,6 +136,7 @@ import { ScheduleMessageModal } from './ScheduleMessageModal';
 import { ScheduledMessagesTray } from './ScheduledMessagesTray';
 import { DraftIndicator } from './DraftIndicator';
 import { scheduledMessagesAtom } from '../../state/scheduledMessages';
+import { createErrorToast, toastQueueAtom } from '../../state/toast';
 import { getThreadDraftKey } from '../../state/room/thread';
 
 const GifPicker = React.lazy(() =>
@@ -184,6 +185,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const [scheduleOpen, setScheduleOpen] = useState(false);
     const [scheduleContent, setScheduleContent] = useState<IContent | null>(null);
     const setScheduledMessages = useSetAtom(scheduledMessagesAtom);
+    const setToast = useSetAtom(toastQueueAtom);
 
     const alive = useAlive();
     // Scope drafts/replies/uploads by thread so a thread composer stays fully
@@ -539,7 +541,22 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       } else if (commandName) {
         const commandContent = commands[commandName as Command];
         if (commandContent) {
-          commandContent.exe(plainText);
+          // Fire-and-forget by design (the editor resets immediately for UX), but
+          // surface a rejection instead of failing silently. NOTE: /kick and /ban
+          // route through rateLimitedActions (utils/matrix.ts), whose to() helper
+          // swallows non-429 errors, so those two commands can still resolve even
+          // when the underlying kick/ban failed — this catch only covers errors
+          // that actually reject out of exe().
+          commandContent.exe(plainText).catch((err) => {
+            console.error(`Failed to run /${commandName} command:`, err);
+            setToast(
+              createErrorToast(
+                `The /${commandName} command failed. Please try again.`,
+                Icons.Warning,
+                'Command failed',
+              ),
+            );
+          });
         }
         resetEditor(editor);
         resetEditorHistory(editor);
@@ -600,6 +617,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       setReplyDraft,
       isMarkdown,
       commands,
+      setToast,
     ]);
 
     /**