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/components/theme.tsx

@@ -0,0 +1,35 @@
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  type ReactNode,
+} from 'react';
+
+type ThemeMode = 'dark' | 'light';
+const KEY = 'msk-theme';
+
+const ThemeContext = createContext<{
+  theme: ThemeMode;
+  setTheme: (t: ThemeMode) => void;
+}>({ theme: 'dark', setTheme: () => {} });
+
+export const useTheme = () => useContext(ThemeContext);
+
+export const ThemeProvider = ({ children }: { children: ReactNode }) => {
+  const [theme, setTheme] = useState<ThemeMode>(() => {
+    if (typeof localStorage === 'undefined') return 'dark';
+    return (localStorage.getItem(KEY) as ThemeMode) || 'dark';
+  });
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+    localStorage.setItem(KEY, theme);
+  }, [theme]);
+
+  return (
+    <ThemeContext.Provider value={{ theme, setTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+};