Add playlist sorting and Spotify import UI

[superthunder-afk]

Summary

This PR adds playlist sorting and Spotify CSV import/export support to the web client.

What changed

• adds sorting controls to normal playlist pages
• fixes playlist page refresh when switching between playlists
• keeps playlist sort controls usable on the normal playlist route
• adds playlist CSV export
• adds Spotify CSV import on playlist pages
• adds Spotify CSV import next to New Playlist on the playlists list page
• adds sorting for favorite tracks

Details

Playlist page

• fetches playlist tracks using the selected sort options
• resets and reloads correctly when moving from one playlist to another
• moves the playlist header controls out of the virtualized song rows so the dropdown is clickable and renders properly

Playlist list

• adds an import button beside New Playlist
• creates a new playlist from the CSV filename, then imports the Spotify CSV into it

Favorites

• adds sort options for favorite tracks
• keeps playback order aligned with the currently selected sort

Requests

• adds blob response handling for CSV downloads
• adds request helpers for playlist export and Spotify CSV import

Notes

• depends on the matching playlist API changes in the SwingMusic server repo

[CLAassistant]

All committers have signed the CLA.