Standalone echo360 executable (SHA: 1272d07)

refactor: replace Selenium with Playwright for browser automation

Remove selenium, undetected-chromedriver, and the binary_downloader module
(ChromeDriver/GeckoDriver downloaders). Replace with Playwright's sync API
across downloader, course, and videos modules.

Simplify CLI: drop --stealth/--chrome flags, keep --firefox for browser choice.
Remove unused utils: naive_versiontuple, BIN_ROOT_PATH.

Standalone echo360 executable (SHA: b06e834)

docs: :memo: update as per readme

Standalone echo360 executable (SHA: 89fc1cf)

fix: :green_heart: remove unneeded args

Standalone echo360 executable (SHA: 2f26a9e)

ci: :construction_worker: mark per commit builds as pre-release builds