Foundation for the isolated video download settings. The Downloads tab is almost
entirely music-specific, so on the video side the music download sections are hidden
(video-side.css) and a data-video-only 'Video Download Folders' section takes their
place — an input (download) and output (transfer/library) folder, stored SEPARATELY
from the music soulseek.* paths in video.db's video_settings KV table.
- api/video/downloads.py: GET/POST /api/video/downloads/config (download_path,
transfer_path), registered in the video blueprint. Imports nothing from music.
- video-settings.js: loadDownloads/saveDownloads wired into onPageShown + the
video save-button chain.
- The shared 'Indexers & Downloaders' tab is left untouched (identical for both).
2 tests (round-trip + the isolation guard). Quality profile, video hybrid, and the
shared slskd block are the next phases.
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