Service ids ('deezer', 'tidal', 'qobuz', 'spotify', 'amazon', 'local')
were compared as raw string literals in every dispatch chain. Introduce
MusicServices constants and use them at the comparison/switch sites so
a typo becomes a compile error instead of a silently dead branch.
legacyProviderIdFromResourceId and stripPrefixedResourceId were
copy-pasted in the album, artist, and playlist screens with slightly
different null semantics; the shared nullable variant plus explicit
?? 'spotify' fallbacks makes the difference visible at the call sites.
zarzet