feat(site): drifting cloud puffs across the public background

Add a fixed, decorative layer of soft clouds that slowly float across
the Habbo sky behind the content:
- Five clouds at staggered sizes, heights, opacities and loop timings
  (60s–190s) so the drift feels organic.
- Dimmed further in dark mode; frozen by prefers-reduced-motion.
- Pure decoration: aria-hidden, pointer-events: none, no color
  utilities in markup (cloud shapes live in globals.css).

src/components/clouds-field.tsx

@@ -0,0 +1,15 @@
+/**
+ * Drifting cloud puffs that float across the public background.
+ * Pure decoration: fixed, behind the content, no pointer events.
+ */
+export function CloudsField() {
+	return (
+		<div className="clouds" aria-hidden="true">
+			<span className="cloud cloud-1" />
+			<span className="cloud cloud-2" />
+			<span className="cloud cloud-3" />
+			<span className="cloud cloud-4" />
+			<span className="cloud cloud-5" />
+		</div>
+	);
+}