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/ui/MarkdownContent.tsx

@@ -43,7 +43,7 @@ export function MarkdownContent({ content, className }: MarkdownContentProps) {
             return (
               <code
                 className={cn(
-                  'block rounded bg-accent p-3 font-mono text-sm overflow-x-auto',
+                  'block rounded bg-code p-3 font-mono text-sm text-foreground overflow-x-auto',
                   className
                 )}
                 {...props}
@@ -54,7 +54,7 @@ export function MarkdownContent({ content, className }: MarkdownContentProps) {
           }
           return (
             <code
-              className="rounded bg-accent px-1.5 py-0.5 font-mono text-sm"
+              className="rounded bg-white/[0.08] px-1.5 py-0.5 font-mono text-sm text-secondary"
               {...props}
             >
               {children}
@@ -63,7 +63,7 @@ export function MarkdownContent({ content, className }: MarkdownContentProps) {
         },
         // Style code blocks (pre)
         pre: ({ children }) => (
-          <pre className="mb-3 overflow-x-auto rounded bg-accent p-0 last:mb-0">
+          <pre className="mb-3 overflow-x-auto rounded bg-code p-0 last:mb-0">
             {children}
           </pre>
         ),