Move `apiWithCredentials` + `restBaseFromUrl` from `api/subsonic.ts`
into `api/subsonicClient.ts` where the other token-auth primitives
already live. They are the credential-bearing variants of `api`,
sitting in the same client module is the natural home.
`api/subsonic.ts` now holds only the connection-probing domain:
`ping`, `pingWithCredentials`, `probeInstantMixWithCredentials`,
`scheduleInstantMixProbeForServer`. Clear, cohesive, stable import
path for the call sites that still use it.
Pure code-move. subsonic.ts: 144 → 119 LOC. Total Phase F journey:
1333 → 119 LOC (~91% reduction).
Frank Stellmacher
e04980626d