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>

backend/package.json

@@ -26,7 +26,6 @@
   "dependencies": {
     "@anthropic-ai/sdk": "^0.72.1",
     "@casl/ability": "^6.8.0",
-    "@casl/prisma": "^1.6.1",
     "@nestjs/axios": "^4.0.1",
     "@nestjs/common": "^10.3.0",
     "@nestjs/config": "^3.1.1",