fix(repo-appearance): clean up color-swatch selection ring sizing
The selected swatch's ring was drawn via a `.padding(2)` inside the
swatch's own frame, so half the stroke landed on top of the colored
fill — visually the ring looked clipped and "fused" to the dot
edge instead of giving the swatch a clean macOS-style halo.
Restructure each swatch into a fixed 28pt slot with a ZStack:
- Color dot: 20pt (slightly tighter than the previous 22pt to leave
room for the ring outside)
- Selection ring: 26pt circle stroked at 1.5pt — drawn outside the
dot inside the slot so a 3pt transparent gap separates dot edge
from ring inner edge, matching macOS native color pickers
- Slot: 28pt fixed width whether or not selected, so changing the
selection doesn't reflow the row of swatches
`noColorSwatch` and the colored swatches share the new
`swatchSlot(isSelected:content:)` helper so the ring chrome stays
consistent across all 11 swatches.