refactor(dashboard,desktop,engine): remove unused imports, props, and locals

Clears the remaining no-unused-vars warnings across the dashboard app and
server, desktop main, and engine sources. Dead React state destructures are
collapsed to setter-only, unused props are underscore-prefixed to preserve
API shape, and unreferenced catch bindings are dropped. No behaviour change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/ChatView.tsx

@@ -464,9 +464,8 @@ export function ChatView({ projectId, addToast }: ChatViewProps) {
   const [mentionStartPos, setMentionStartPos] = useState(-1);
 
   // File mention state and hook
-  const [fileMentionPopupVisible, setFileMentionPopupVisible] = useState(false);
+  const [, setFileMentionPopupVisible] = useState(false);
   const [fileMentionPosition, setFileMentionPosition] = useState({ top: 0, left: 0 });
-  const fileMentionRef = useRef<HTMLDivElement>(null);
 
   const fileMention = useFileMention({ projectId });