Replaces the fire-and-forget button with a premium modal that shows
exactly which artists will be added before confirming. Features:
- Glassmorphic modal with stat cards, two-column artist grid, search
filter, collapsible ineligible section, and loading spinner
- Source-aware filtering: only shows artists with the active source's
ID (Spotify/iTunes/Deezer) as eligible
- Frontend and backend both paginate at 400 to avoid SQLite variable
limit (SQLITE_MAX_VARIABLE_NUMBER=999) that silently broke queries
above ~500 artists
- Backend source detection aligned with frontend — uses only the
active source's ID, falls back to configured metadata source
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