refactor(core): derive color param names from a single source of truth

[marcalexiei]

• Related to support light & dark mode in one card #430 (review)

Derive the color param names from one source of truth.

They were spelled out in different places:

1. the ColorInput interface
2. the extractLightDarkColors mapping,
3. 24-entry COLOR_PARAM_KEYS array,
4. theme-exclusion literal repeated in all five api/*.js files.

Adding a param meant touching all four, with nothing to catch a miss.

Now everything derives from:

const BASE_COLOR_KEYS = [/* ... */ ] as const;

const THEME_VARIANTS = ["light", "dark"] as const;

ColorInput and LightDarkColorParams become mapped types over those keys, extractLightDarkColors maps over BASE_COLOR_KEYS, and COLOR_PARAM_KEYS is built by flat-mapping the variants.

The api files each repeated the same "theme params aren't colors" exclusion:

findInvalidColor({ ...colorParams, theme: undefined, theme_light: undefined, theme_dark: undefined })

That's now findInvalidColorParam(colorParams), with the exclusion derived in color.ts.

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[martin-mfg]

Why add | undefined here?

[marcalexiei]

To allow undefined to be passed explicitly: