phsl / new-api

Mitigate XSS vulnerabilities in the playground where AI-generated content
is rendered without sanitization, allowing potential script injection via
prompt injection attacks.

MarkdownRenderer.jsx:
- Replace dangerouslySetInnerHTML with a sandboxed iframe for HTML preview
- Use sandbox="allow-same-origin" to block script execution while allowing
  CSS rendering and iframe height auto-sizing
- Add SandboxedHtmlPreview component with automatic height adjustment

CodeViewer.jsx:
- Add escapeHtml() utility to encode HTML entities before rendering
- Rewrite highlightJson() to process tokens iteratively, escaping each
  token and structural text before wrapping in syntax highlighting spans
- Escape non-JSON and very-large content paths that previously bypassed
  sanitization
- Update linkRegex to correctly match URLs containing & entities

These changes only affect the playground (AI output rendering). Admin-
configured content (home page, about page, footer, notices) remains
unaffected as they use separate code paths and are within the trusted
admin boundary.