feat(FN-839): add nord, dracula, gruvbox, and tokyo-night color themes

- Add new theme IDs (nord, dracula, gruvbox, tokyo-night) to core types and ThemeSelector component
- Add CSS custom properties for all four themes covering background, text, accent, border, and surface colors
- Add theme validation sync, bootstrap logic, and unit tests for ThemeSelector and useTheme hook
- Update GitManagerModal to support new theme-aware styling
- Update README theme system documentation with expanded theme list
- Add changeset for published package

packages/core/src/types.ts

@@ -24,6 +24,10 @@ export const COLOR_THEMES = [
   "factory",
   "ayu",
   "one-dark",
+  "nord",
+  "dracula",
+  "gruvbox",
+  "tokyo-night",
 ] as const;
 export type ColorTheme = (typeof COLOR_THEMES)[number];