Refine MV3 export workflow

[YBloom]

Summary

• update the MV3 manifest to load local libraries, apply the new injected stylesheet, and target ChatGPT/Gemini hosts
• rebuild the content script to inject export actions after completion, prefer markdown-to-PDF rendering, and fall back to DOM capture with reliable pagination
• add styling for injected menu items to blend with the native ellipsis menu
• inline iframe libraries for PDF export, extend timeouts, and bridge jsPDF to prevent sandbox load failures

Testing

• not run (manual testing pending)

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Codex Task