fix: restore library tracks via fetchLibraryTracks

[masonc15]

Problem

spogo library tracks list could return an empty success from the Connect client.

The previous implementation queried libraryV3 with the Songs filter. In current Spotify responses, that request behaves like an invalid liked-songs path: the payload can include LibraryInvalidFilterIdError while still decoding to zero items, so the web API fallback never activates.

Liked songs are available through fetchLibraryTracks for spotify:collection:tracks, but that operation returns a different shape: data.me.library.tracks.items[].track.data, with _uri on the track wrapper rather than inside data.

Changes

• Switch libraryTracks() from libraryV3 to fetchLibraryTracks with spotify:collection:tracks.
• Add a dedicated extractor for the fetchLibraryTracks payload and inject _uri into the extracted track data before mapping.
• Treat missing or malformed fetchLibraryTracks payloads as errors so LibraryTracks() falls back to the web API instead of silently returning an empty result.
• Tighten tests to validate the fetchLibraryTracks request variables and cover the fallback path when the response shape drifts.

Testing

• go test ./...
• Manually verified spogo library tracks list --limit 5
• Manually verified spogo library tracks list --limit 3 --json