chore: Remove deprecated js config for Tailwind

Tailwind v4 moves from tailwind.config.mjs to using the @theme directive in css:
https://tailwindcss.com/docs/upgrade-guide#using-a-javascript-config-file

There is currently no way to cleanly reference the default font stack without copy-pasting in Tailwind v4:
tailwindlabs/tailwindcss#15253

Author: thomas-kuntz

src/styles/global.css

@@ -1,6 +1,12 @@
 @import "tailwindcss";
 @plugin "@tailwindcss/typography";
-@config "../../tailwind.config.mjs";
+
+@theme {
+  --font-sans: "Inter", ui-sans-serif, system-ui, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
+  --font-serif: "Lora", ui-serif, Georgia, Cambria, "Times New Roman", Times, serif;
+}
+
+@custom-variant dark (&:where(.dark, .dark *));
 
 html {
   overflow-y: scroll;