feat(FN-995): add slate, ash, graphite, silver color themes

- Add four new neutral-palette themes (slate, ash, graphite, silver) to the theme type definitions and selector
- Add full CSS variable sets for each theme covering backgrounds, text, borders, accents, and UI element colors
- Add swatch preview entries in ThemeSelector component for the new themes
- Add comprehensive tests covering theme registration, selector rendering, and CSS variable application

packages/core/src/types.ts

@@ -20,6 +20,10 @@ export const COLOR_THEMES = [
   "high-contrast",
   "industrial",
   "monochrome",
+  "slate",
+  "ash",
+  "graphite",
+  "silver",
   "solarized",
   "factory",
   "ayu",