fix(edit): curate the CalDAV colour picker in painted space (#22)

The picker paints every swatch through pastelize, which pins lightness to
a constant and caps saturation — so the raw palette's lightness axis is
invisible on screen. Curating in raw space (as the first pass did) left
near-identical painted swatches (navy/blue/midnightblue all paint as one
mid blue) and stranded every neutral as an identical pale tint in a run of
"pinks" at the end of the grid.

Move dedup, thinning, and ordering into the painted (pastelised) space the
user actually sees: colours that paint identically collapse to one,
oversized palettes drop washed-out neutral-origin tints (painted chroma
below a floor) and thin by CIE76 ΔE in painted Lab, and the survivors sort
continuously by painted hue with the wheel cut at its widest gap. The CSS3
147-colour dump now lands at ~33 distinct, rainbow-ordered swatches with no
neutrals stranded at the end.

The hue/saturation shaping moves into a shared domain pastelArgb() so the
picker paints exactly what curation reasons about; ui/pastelize wraps it and
only picks the theme's brightness.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -28,7 +28,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   app (such as DAVx5), the event colour picker showed every colour the account
   publishes — nearly 150 swatches in alphabetical order, many of them
   duplicates or near-identical shades. The picker now shows only visually
-  distinct colours, arranged as a rainbow with the grays at the end. Picked
+  distinct colours, arranged as a rainbow; near-duplicate shades and the
+  washed-out neutrals are folded away so no two swatches look alike. Picked
   colours still sync exactly as before, and calendars with hand-picked
   palettes (like Google's) are unaffected. Thanks to @ptab for the report
   ([#22]).