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>

tsup.config.ts

@@ -0,0 +1,13 @@
+import { defineConfig } from 'tsup';
+
+export default defineConfig({
+  entry: { index: 'src/index.ts' },
+  format: ['esm', 'cjs'],
+  dts: true,
+  clean: true,
+  treeshake: true,
+  sourcemap: true,
+  // react/react-dom come from the host app; radix-ui + phosphor are
+  // declared dependencies and are externalized automatically by tsup.
+  external: ['react', 'react-dom'],
+});