feat: Upgrade to v2.0 with major download stability overhaul

This commit marks a major upgrade to version 2.0, focusing entirely on resolving the critical stability issues encountered during parallel downloads.

Key changes include:

- **New Stable Download Mechanism:** Replaced the previous download logic with a manual, chunk-based implementation. This introduces configurable delays (`chunk_delay_ms`) between chunks, effectively preventing `GetFileRequest` flood waits and enabling truly stable parallel downloads.

- **Resolved Expired File References:** Workers now fetch a fresh file reference on-demand just before a download begins. This completely resolves the `File reference has expired` error, which previously caused downloads to fail and create 0-byte files.

- **Intelligent State Management:** The state persistence logic has been reworked to track the last successfully *downloaded* file's message ID, rather than the last scanned ID. This ensures no pending downloads are lost if the script is restarted.

- **Comprehensive Code & Config Refactor:** Performed a major cleanup by removing a significant amount of dead code, unused functions, and obsolete settings. The `config.json` structure has also been refactored to be more intuitive, with clear `premium_settings` and `free_settings` blocks.

- **Full Documentation Update:** The `CHANGELOG.md`, `README.md`, and `QUICK_START.md` files have been completely updated to reflect all the new features, the new configuration structure, and the improved stability of version 2.0.

Author: challal