Rework the color palette for vividness and range

[danielchalmers]

What changed

The widget color presets were washed out (backgrounds at 98% lightness, dark desaturated accents) and clustered — three near-identical greens, rose/coral overlapping, and no yellow or magenta at all.

• Less dull: light-mode backgrounds now carry a real wash of color, borders are visible, and accents sit at full saturation so progress bars, habit dots, and the swatch picker pop. Dark mode gets richer tinted cards and brighter accents.
• Better range: hues are retuned to spread evenly around the wheel, and two new presets — Lemon and Fuchsia — fill the yellow and magenta gaps (12 total).
• No migration needed: all ten existing preset ids are kept, so stored boards keep their chosen colors.
• The theme picker lays the twelve swatches on a single non-wrapping row in spectral order; they shrink evenly in narrow dialogs instead of spilling to a second row.

Verified by loading the built extension in Chromium and screenshotting a board seeded with every preset in light and dark mode; unit tests and typecheck pass.