fix: sanitize exported chat HTML content

[aravelo7]

What this PR does / why we need it?

This PR sanitizes exported chat HTML content before generating downloadable .html files.

Previously, exported HTML content was generated directly from chat markdown via marked(...) without sanitization. Since exported content may contain user messages and assistant responses, raw HTML or dangerous URL schemes could persist into the exported file.

This change reduces potential export-based XSS risks in exported chat HTML files.

Summary of your change

• Sanitize exported HTML using sanitize-html

• Preserve common Markdown export elements:

  • headings
  • paragraphs
  • lists
  • blockquotes
  • code blocks
  • tables
  • links
  • images

• Restrict dangerous tags, event handler attributes, and unsafe URL schemes

• Keep the change isolated to the HTML export flow

• Add minimal TypeScript declaration for sanitize-html

• Update marked(markdownContent) to await marked(markdownContent) for current type compatibility

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of [Conventional Commits specification](https://www.conventionalcommits.org/).
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[f2c-ci-robot]

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