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>

src/lib/utils.ts

@@ -0,0 +1,6 @@
+import { clsx, type ClassValue } from "clsx";
+import { twMerge } from "tailwind-merge";
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}