Refine PS1/PS2/PS3/NDS themes and add decorative background field

Rework console theme stylesheets and add opt-in .rb-bg cube field
(used by PS2 BIOS bobbing-cube background) wired through Shell.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/Shell.tsx

@@ -1,5 +1,5 @@
 import Link from "next/link";
-import type { ReactNode } from "react";
+import type { CSSProperties, ReactNode } from "react";
 import { THEMES, type ThemeId } from "@/themes/registry";
 import { getSettings } from "@/lib/settings";
 import { isAdmin } from "@/lib/auth";
@@ -29,6 +29,30 @@ export default async function Shell({
 
   return (
     <div className="rb-desktop">
+      {/* Decorative background field. Hidden by default; themes opt in
+          (PS2 uses it for the BIOS bobbing-cube field). */}
+      <div className="rb-bg" aria-hidden>
+        <div className="rb-bg-grid">
+          {Array.from({ length: 15 * 13 }).map((_, i) => {
+            // deterministic pseudo-random thickness + bob phase per cube
+            const h = 34 + ((i * 53) % 78);
+            const delay = (((i * 37) % 100) / 100) * 4;
+            return (
+              <span
+                key={i}
+                className="rb-cube"
+                style={
+                  {
+                    "--cube-h": `${h}px`,
+                    "--cube-delay": `${delay.toFixed(2)}s`,
+                  } as CSSProperties
+                }
+              />
+            );
+          })}
+        </div>
+      </div>
+
       <div className="rb-window" role="application">
         <div className="rb-titlebar">
           <span className="rb-titlebar-icon" aria-hidden />