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>

components.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "default",
+  "rsc": true,
+  "tsx": true,
+  "tailwind": {
+    "config": "tailwind.config.ts",
+    "css": "app/globals.css",
+    "baseColor": "zinc",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  }
+}