Switch dark mode from class-based to CSS media queries

- tailwind.config.ts: darkMode "class" → "media"
- globals.css: .dark { } → @media (prefers-color-scheme: dark) { :root { } }

All dark: utility classes in components work automatically with either mode.
No toggle UI existed, so nothing else to remove.

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

tailwind.config.ts

@@ -4,7 +4,7 @@ import typography from "@tailwindcss/typography";
 import tailwindAnimate from "tailwindcss-animate";
 
 export default {
-  darkMode: "class",
+  darkMode: "media",
   content: ["./src/**/*.tsx", "./src/**/*.mdx"],
   theme: {
     extend: {