- Implement `getLemonSqueezyEnv` function to validate required environment variables for Lemon Squeezy integration using `zod`.
- Ensure `LEMON_SQUEEZY_SECRET_KEY`, `LEMON_SQUEEZY_SIGNING_SECRET`, and `LEMON_SQUEEZY_STORE_ID` are present and valid.
Updated the billing system's schema to change 'storeId' to a string type, and improved the cleanliness and readability of the code. Enhanced the logging system within the billing service for better tracking and debugging. In line with these changes, added corresponding error pages in the client side to handle any errors.