refactor: restructure configuration and enhance proxy checking logic

- Updated configuration structure to better organize proxy and subscription settings, including new fields for proxy check interval, IP check URL, and status check URL.
- Refactored main application logic to utilize the new configuration structure, improving clarity and maintainability.
- Enhanced proxy checking methods to support new configurations and improved error handling.
- Updated web handlers and templates to reflect changes in configuration and provide accurate metrics display.
- Improved logging for subscription updates and proxy checks, ensuring better traceability of operations.