Fix theme toggle using system preference instead of selected theme (#326)

Tailwind v4 defaults dark: utilities to @media (prefers-color-scheme)
strategy. Add @custom-variant to use class-based dark mode matching
the next-themes ThemeProvider attribute="class" configuration.

Fixes https://github.com/manaflow-ai/cmux/issues/325

web/app/globals.css

@@ -1,5 +1,7 @@
 @import "tailwindcss";
 
+@custom-variant dark (&:where(.dark, .dark *));
+
 :root {
   --background: #fafafa;
   --foreground: #171717;