chore: remove unused packages, imports, and stale type definitions

- Remove @casl/prisma (unused) from backend
- Remove @heroicons/react (unused, using lucide-react) from frontend
- Remove unused InferSubjects import from ability.factory.ts
- Remove unused Calendar import from Dashboard.tsx
- Delete stale frontend/src/lib/types.ts (duplicate of src/types/index.ts)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/package.json

@@ -18,7 +18,6 @@
     "@auth0/auth0-react": "^2.2.4",
     "@casl/ability": "^6.8.0",
     "@casl/react": "^5.0.1",
-    "@heroicons/react": "^2.2.0",
     "@react-pdf/renderer": "^4.3.2",
     "@tanstack/react-query": "^5.17.19",
     "axios": "^1.6.5",