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/GitManagerModal.tsx

@@ -168,7 +168,7 @@ interface GitManagerModalProps {
 
 // ── Main Component ────────────────────────────────────────────────
 
-export function GitManagerModal({ isOpen, onClose, tasks, addToast, projectId }: GitManagerModalProps) {
+export function GitManagerModal({ isOpen, onClose, tasks: _tasks, addToast, projectId }: GitManagerModalProps) {
   const [activeSection, setActiveSection] = useState<SectionId>("status");
   const [loading, setLoading] = useState(false);
   const [sectionError, setSectionError] = useState<string | null>(null);