Create and edit playlist modals now upload/preview a cover image
(reusing the presigned-upload flow already used for tracks) and share
identical layout and coloring. Adds a cover proxy route and wires
PlaylistArt to render a playlist's real cover before falling back to
the track mosaic/gradient.
Also reworks the playlist detail page: full-width borderless "Añadir
de tu biblioteca" table with listens count and click-to-play, drops
the empty-playlist placeholder message, and restyles the playlists
grid to match the rest of the app.
Move authenticated pages under an (app) group with a Sidebar shell, add /auth/login and page-based /login, drop the old explore page and login +server endpoint, and add the server upload helper.
Admin