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/components/tree-editor/NodeEditorModal.tsx

@@ -86,7 +86,7 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
     <Modal isOpen={true} onClose={onClose} title={getTitle()} size="lg" footer={footerContent} allowFullScreen={true}>
       {/* Node ID display */}
       <div className="mb-4 text-xs text-muted-foreground">
-        Node ID: <code className="rounded bg-accent px-1 py-0.5">{node.id}</code>
+        Node ID: <code className="rounded bg-white/[0.08] px-1 py-0.5">{node.id}</code>
       </div>
 
       {/* Validation errors */}

frontend/src/components/tree-editor/code-mode/SyntaxHelpPanel.tsx

@@ -103,7 +103,7 @@ function Row({ label, code }: { label: string; code: string }) {
   return (
     <div className="flex items-center justify-between py-0.5">
       <span className="text-muted-foreground">{label}</span>
-      <code className="rounded bg-accent px-1 py-0.5 text-[10px] text-muted-foreground">{code}</code>
+      <code className="rounded bg-white/[0.08] px-1 py-0.5 text-[10px] text-muted-foreground">{code}</code>
     </div>
   )
 }