- Add yt-url input, FETCH button, and yt-progress span to transport bar
- Add fetchWithProgress() SSE ReadableStream reader function
- Add renderProgressBar() terminal-style block fill renderer (DOWNLOADING ████)
- Wire FETCH button click handler with D-02 disable-during-download behavior
- Auto-load audio into player after download (D-01 same pipeline as upload)
- Add .yt-url-input and .yt-progress CSS styles (flat, monospace, no border-radius)
- Enable yt-url input on page load via JS