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/hooks/useModalManager.ts

@@ -121,7 +121,7 @@ export interface ModalManager {
  * and cross-modal transitions (for example, script runner -> terminal handoff).
  */
 export function useModalManager(options: UseModalManagerOptions): ModalManager {
-  const { projectId, planningSessions } = options;
+  const { planningSessions } = options;
 
   const [newTaskModalOpen, setNewTaskModalOpen] = useState(false);
   const [isPlanningOpen, setIsPlanningOpen] = useState(false);