Add Discogs artwork selection UI in album edit modal

Backend:
- Add DiscogsImageOption struct for artwork options
- Add discogs_search_artwork command to search multiple images
- Add discogs_download_artwork command to download selected image
- Extend DiscogsClient with search_artwork_options() method
- Extend DiscogsClient with download_artwork_from_url() method
- Register new commands in lib.rs

Frontend:
- Add Discogs image selection to album edit modal
- Add 'Fetch from Discogs' button with Discogs logo
- Display grid of artwork options from Discogs search results
- Allow user to select preferred artwork option
- Download and save selected artwork on Save
- Add comprehensive CSS styling for Discogs UI elements
- Adjust Discogs logo color filter for proper contrast

User can now click 'Fetch from Discogs', browse multiple artwork
options, select one, and save it as the album's cover art.

static/discogs_icon.svg

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