feat(rithmomachia): add standalone guide page route

Adds /arcade/rithmomachia/guide route that displays the playing guide
modal in standalone mode. This allows the guide to be opened in a
separate window/tab.

Route: /arcade/rithmomachia/guide
Component: PlayingGuideModal with standalone=true

Fixes 404 error on production for this route.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/app/arcade/rithmomachia/guide/page.tsx

@@ -0,0 +1,22 @@
+'use client'
+
+import { useState } from 'react'
+import { PlayingGuideModal } from '@/arcade-games/rithmomachia/components/PlayingGuideModal'
+
+export default function RithmomachiaGuidePage() {
+  // Guide is always open in this standalone page
+  const [isOpen] = useState(true)
+
+  return (
+    <div
+      style={{
+        width: '100vw',
+        height: '100vh',
+        overflow: 'hidden',
+        background: '#f3f4f6',
+      }}
+    >
+      <PlayingGuideModal isOpen={isOpen} onClose={() => window.close()} standalone={true} />
+    </div>
+  )
+}