Feature: Add Chrome support, rate-limit retry logic, and update docs

[theFuribundi]

This PR refactors substack_scraper.py to add cross-browser support and improve stability when scraping large archives.

Key Changes:

• Dual Browser Support: Added support for Google Chrome via webdriver_manager. The script now accepts a --browser argument (chrome, edge, or auto) and attempts to auto-detect the available browser if one fails.
• Rate Limit Handling: Implemented exponential backoff and retry logic. If a 429 Too Many Requests or empty template is detected, the script now waits and retries instead of crashing or skipping the post.
• Smart Skipping: Moved the sleep timer inside the existence check. The script now verifies if a local file exists before pausing, allowing it to "fast-forward" through already downloaded posts without unnecessary delays.
• Human-like Behavior: Replaced fixed sleep timers with randomized "jitter" (10-20s) and added wait times for JS rendering to prevent detection and ensure content loads fully.
• Improved Logging: Switched print statements to tqdm.write to prevent interference with the progress bar.
• Documentation: Updated README.md with the correct clone URL and ensured requirements.txt includes webdriver_manager.

Tested on Ubuntu 24.04.