fix: prevent white flash on page load when color scheme is auto

apps/nextjs/src/app/[locale]/layout.tsx

@@ -4,6 +4,7 @@ import { Inter } from "next/font/google";
 import "@homarr/notifications/styles.css";
 import "@homarr/spotlight/styles.css";
 import "@homarr/ui/styles.css";
+import "~/styles/color-scheme.scss";
 import "~/styles/scroll-area.scss";
 
 import { notFound } from "next/navigation";

apps/nextjs/src/styles/color-scheme.scss

@@ -0,0 +1,12 @@
+// Prevent flash of white background when color scheme is "auto" and OS prefers dark.
+// CSS media queries are evaluated synchronously during the browser's initial paint,
+// before any JavaScript executes, so this eliminates the white→dark flicker.
+html[data-mantine-color-scheme="auto"] {
+  background-color: #fff;
+}
+
+@media (prefers-color-scheme: dark) {
+  html[data-mantine-color-scheme="auto"] {
+    background-color: #242424;
+  }
+}