Phase 3 of the Search/Artists unification. The Search page's two-mode
toggle is replaced by a single 'Search from' dropdown: All sources
(Auto), Spotify, Apple Music, Deezer, Discogs, Hydrabase, MusicBrainz,
or Soulseek (raw files). Auto keeps today's fan-out behavior for
backwards compatibility; picking a specific source hits only that
provider. 'Soulseek' routes to the raw-file basic section, so one
picker covers both old modes. Loading text and the enhanced fetch
now respect the selected source. Zero API changes — uses the source
param added in 2.40 and the shared fetch helper from 2.41.
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