Convert app into @modernsk/ui component library package

- Add library entry (src/index.ts) re-exporting all components, theme,
  and TooltipProvider
- Add shippable stylesheet (src/styles/index.css): tokens + components
  only, font inlined as base64 at build time
- Build with tsup (ESM + CJS + .d.ts) and esbuild for CSS
- package.json: exports map, files, sideEffects, peerDependencies
  (react/react-dom), correct deps (radix-ui), prepare-on-install
- Fix phantom dependency: declare radix-ui, drop unused @radix-ui/themes
- Remove Storybook boilerplate, Tailwind/PostCSS (unused)
- Keep App.tsx + Rsbuild as dev-only playground (not published)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/index.ts

@@ -0,0 +1,12 @@
+/* ============================================================
+   ModernSK UI — public package entry.
+   Import components from here; import the stylesheet once at your
+   app root:  import '@modernsk/ui/styles.css';
+   ============================================================ */
+import { Tooltip } from 'radix-ui';
+
+export * from './components/ui';
+export { ThemeProvider, useTheme } from './components/theme';
+
+/* Tooltips need a single provider near your app root. */
+export const TooltipProvider = Tooltip.Provider;