feat(accent): pick the colour that stands out, not the average

colorz returned cluster means, which averaged a small vivid element into
its surroundings, so a violet motif on a frame of pale skin came out as a
muddy red. signature_color() now reads the pixels of a 256px thumbnail
itself and scores each by chroma times its distance from the image's mean
colour. The top 2% vote by hue, neighbouring bands summed, and the most
vivid pixel near the peak is snapped. A pick under chroma 25 is a tint and
takes the fallback.

Dropping colorz also fixes two bugs. Its k-means seeds are random, so one
wallpaper could snap differently between runs. And it calls sys.exit() on
some images, which `except Exception` in main() does not catch, so
udt-accent died without writing and the previous accent stayed.

Over 314 wallpapers: no errors (colorz failed on 18), 25 fall back,
about 0.06s each. The selftest gains a small-vivid-patch case and a
grey-image case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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