Section headers lose the bracket look (no accent bar, no rounded corners)
and span the full tile width, with text still aligned to the other rows.
A "Colors" group in the gear menu holds swatch pickers for the song key,
section headers and the current item, a switch for fading the header color
to the right (on by default), and the item-type tint. Colors apply as CSS
custom properties on the tile.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
