Fix recording freeze: handle queued mode in stop endpoint

Fix HF test: allow real username when huggingface_hub is installed

Separate local and downloaded datasets: block recording into downloads

Add HF auto-login detection: auto-fill username from CLI session

Add 'starting_up' mode for non-blocking startup animation

Remove denoise feature from UI and backend

Fix P0-1: Replace fragile progress bar timing with server-driven phases

Add test infrastructure: 50 unit tests + 13 Playwright E2E tests