fix: replace bg-accent with neutral backgrounds on code/kbd elements

bg-accent maps to bright cyan (#22d3ee) in Tailwind v4 — wrong for
code elements and keyboard shortcut badges. This was a recurring issue
across the app where every backticked term glowed cyan.

- Code blocks: bg-accent → bg-code (#14161e)
- Inline code: bg-accent → bg-white/[0.08] (subtle neutral tint)
- Kbd badges: bg-accent → bg-white/[0.08]
- AI chat markdown: same fixes in MarkdownContent.tsx

Cyan remains reserved for interactive elements (buttons, active nav,
links) per the design system.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/admin/InviteCodesPage.tsx

@@ -119,7 +119,7 @@ export function InviteCodesPage() {
       key: 'code',
       header: 'Code',
       render: (c) => (
-        <code className="rounded bg-accent px-2 py-1 text-sm font-mono text-muted-foreground">{c.code}</code>
+        <code className="rounded bg-white/[0.08] px-2 py-1 text-sm font-mono text-muted-foreground">{c.code}</code>
       ),
     },
     {