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>

tsconfig.json

@@ -24,7 +24,8 @@
     ],
     "paths": {
       "@/*": [
-        "./*"
+        "./*",
+        "src/*"
       ],
       "@/src/*": [
         "src/*"