Fix API key leak security issue

[vthish]

This pull request updates how Firebase configuration is handled in the frontend_apps/auto_ledger project. Instead of hardcoding sensitive Firebase credentials in firebase_options.dart, the code now loads them from environment variables using the flutter_dotenv package. This improves security and flexibility for different environments (e.g., development, production).

Key changes:

Security and Configuration Management

• Replaced hardcoded Firebase credentials in the DefaultFirebaseOptions class with dynamic values loaded from environment variables via the flutter_dotenv package. This applies to both Android and iOS configurations, which are now accessed through getters instead of constants.

Dependency Updates

• Imported the flutter_dotenv package in firebase_options.dart to enable loading environment variables for Firebase configuration.