fix(gtk): derive the twelve computed tints from the palette too

Twelve hexes were shades upstream computed from a palette colour rather than
taking one. They were left literal on the same reasoning as the neutral greys,
which was wrong: a grey is shading, but each of these paints one widget
surface in a colour that is supposed to be the theme's. Under any scheme but
Macchiato they stayed Macchiato, including the window close, minimise and
maximise hover fills.

They are substituted as @ROLE_PCT@ now, the percentage being that shade's
brightness relative to its base. shade() scales each channel and clamps, so it
keeps the relationship rather than the exact upstream value; closest on the
accent family, crudest on the reds, where upstream darkened toward saturation.

GTK4 carries ten of the twelve: the selected progress bar and the
selection-mode titlebar backdrop are GTK3-only widgets upstream.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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