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/MaintenanceFlowDetailPage.tsx

@@ -193,7 +193,7 @@ export default function MaintenanceFlowDetailPage() {
                   : <AlertCircle className="h-3 w-3" />}
                 {schedule.is_active ? 'Active' : 'Paused'}
               </span>
-              <code className="rounded bg-accent px-1.5 py-0.5 text-[0.8125rem] text-foreground">
+              <code className="rounded bg-white/[0.08] px-1.5 py-0.5 text-[0.8125rem] text-foreground">
                 {schedule.cron_expression}
               </code>
               <span className="text-muted-foreground">({schedule.timezone})</span>