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/useFileMention.ts

@@ -210,7 +210,7 @@ export function useFileMention(options: UseFileMentionOptions = {}): UseFileMent
    * Supports ArrowUp/ArrowDown to navigate, Enter/Tab to select, Escape to dismiss.
    */
   const handleKeyDown = useCallback(
-    (event: React.KeyboardEvent<HTMLElement>, currentText: string): boolean => {
+    (event: React.KeyboardEvent<HTMLElement>, _currentText: string): boolean => {
       if (!mentionActive || files.length === 0) {
         return false;
       }