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

data/.gitignore

@@ -0,0 +1,5 @@
+# Exclude all user data files
+*.json
+
+# Keep this .gitignore file
+!.gitignore