feat: scaffold tailwind + shadcn foundation, swap to next-themes

Installs Tailwind CSS v3, postcss, autoprefixer, next-themes, lucide-react,
clsx, tailwind-merge, class-variance-authority, sonner, and tailwindcss-animate.
Creates tailwind.config.ts, postcss.config.mjs, components.json, src/lib/utils.ts
(cn helper), replaces globals.css with CSS variable theme, adds
suppressHydrationWarning to html element, and replaces MUI ThemeProvider
with next-themes ThemeProvider + sonner Toaster. MUI remains installed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

postcss.config.mjs

@@ -0,0 +1,8 @@
+const config = {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};
+
+export default config;