feat(web/api): playlists helper + types + query keys (M7 #352 slice 1)

Wire shapes mirror the server's snake_case envelope. URL helpers use
the existing apiFetch + ApiError surface so error codes (not_found,
not_authorized, bad_request, server_error) propagate via the same
copyForCode chain as everything else.

web/src/lib/api/queries.ts

@@ -38,6 +38,8 @@ export const qk = {
   home:         ()                 => ['home'] as const,
   albumsAlpha:  ()                 => ['albumsAlpha'] as const,
   artistTracks: (artistId: string) => ['artistTracks', artistId] as const,
+  playlists:    ()                 => ['playlists'] as const,
+  playlist:     (id: string)       => ['playlist', id] as const,
 };
 
 export function createArtistsQuery(sort: ArtistSort) {