The native <input type="color"> only supports 6-digit hex, stripping alpha
and forcing background/border/text colors to be fully opaque on edit.
Each color field now shows an opacity slider (0–100%) below the swatch.
Values are stored as 8-digit hex (#rrggbbaa). Existing zones with 6-digit
colors are handled transparently (alpha defaults to 100%).
- colorUtils.ts: hexToRgba / rgbaToHex8 helpers
- GroupRectModal: opacity sliders for all three color fields
- 26 new tests across colorUtils and GroupRectModal
Pouzor