feat(theme-selector): custom in-page color picker

I replaced the native OS color swatch with an in-page picker, since the native one couldn't be sized or restyled and had no clean way to apply and dismiss. Clicking the swatch opens a popup with a saturation/value square and a hue slider you drag, a live hex plus AAA/AA/FAIL contrast readout, and the current palette as clickable chips for grabbing an exact existing color. It writes to the hex field, so add and update work unchanged, and it closes on click-away or when you commit a color.

The HSV math is self-contained and the contrast readout reuses the existing helpers, so there's no dependency. A hash-guarded #pick hook opens the picker for headless screenshot verification, the same pattern as the #selftest harness.

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