Add subtle radial gradient to app background

Apply a soft blue radial glow centered on the viewport to add depth
to the dark background, replacing the flat solid color.

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

apps/web/src/index.css

@@ -198,6 +198,12 @@
 
 body {
 	background-color: var(--color-background);
+	background-image: radial-gradient(
+		ellipse at 50% 40%,
+		oklch(0.26 0.055 250) 0%,
+		var(--color-background) 70%
+	);
+	background-attachment: fixed;
 	color: var(--color-foreground);
 	font-family: var(--font-sans);
 }