Migrate from Firebase to file-based storage system

- Remove Firebase dependency and all Firebase code
- Implement local file-based storage with encryption
- Create custom authentication system with password hashing
- Add API routes for auth (register/login) and data operations
- Create FileStorageService for server-side file I/O
- Add AuthService for client-side authentication wrapper
- Update all components to use new storage system
- Add comprehensive documentation (FILE_STORAGE.md, MIGRATION_SUMMARY.md)
- Improve security with AES-GCM encryption and SHA-256 password hashing
- Add ErrorBoundary component for better error handling
- Fix multiple bugs: date serialization, transaction deletion, envelope calculations

package.json

@@ -9,7 +9,6 @@
     "lint": "eslint"
   },
   "dependencies": {
-    "firebase": "^12.4.0",
     "next": "16.0.0",
     "react": "19.2.0",
     "react-dom": "19.2.0"