feat(theme): hex-aware From Colors wizard + theme grid scroll + FontAwesomeIcon explicit re-export

- ColorHarmonyPicker rewrite: hex picker (categorized swatches + custom hex), Tetradic + Monochromatic harmonies, HSL nudge sliders, 4-channel palette preview, always emits hex
- themeGenerator: hex-mode generateHarmonyTheme with inlined harmony math (complementHex, analogousHexes, triadicHexes, splitComplementaryHexes, tetradicHexes, monochromaticHexes, adjustHsl)
- ThemePickerGridPane: themes grid now actually scrolls (flex-1 min-h-0)
- Explicit FontAwesomeIcon re-export from dash-react (helps webpack's static analyzer through symlinked builds)