fix(web): add Tailwind CSS setup for design system rendering

The login page design components use Tailwind utility classes but
tailwindcss, postcss, and autoprefixer were never added to the web
app's dependencies. Without these, @tailwind directives in globals.css
produce no output and all utility classes render as plain strings.

Adds:
- tailwindcss, postcss, autoprefixer as devDependencies
- tailwind.config.ts with dark mode class strategy and UI package
  content scanning
- postcss.config.mjs with tailwindcss + autoprefixer plugins

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

apps/web/postcss.config.mjs

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