Feature Request: Add export and copy functionality for assistant responses

### What

Feature Request: Add export and copy functionality for assistant responses

I would like WordPress Studio to include export options for assistant responses, similar to other AI interfaces. Specifically:

- Export to PDF format
- Export to Markdown (.md) format
- Export to DOCX format
- A "Copy All" button to copy the entire response with proper formatting

Currently, users can only copy responses manually by selecting text, which doesn't preserve formatting well and is cumbersome for long responses.

### Why

This feature would greatly improve the user experience by allowing users to:

- Save important responses for offline reference
- Share assistant responses with team members who don't have access to WordPress Studio
- Build documentation from helpful responses
- Maintain proper code formatting when copying technical responses
- Create training materials or knowledge bases from assistant interactions

### How

Implementation suggestions:

1. Add an export dropdown menu to each assistant response (similar to the attached mockup)
2. Include format options: PDF, Markdown, DOCX
3. Add a dedicated "Copy All" button that preserves formatting
4. For PDF export: Include syntax highlighting for code blocks
5. For Markdown: Preserve all formatting including code blocks, lists, and headers
6. For DOCX: Convert markdown formatting to Word styles

Technical approach could use existing libraries like:
- jsPDF for PDF generation
- markdown-pdf or similar for conversions
- Proper clipboard API for enhanced copy functionality

Start by locating the assistant-response UI and its existing copy behavior in the WordPress Studio desktop app. Define how PDF, Markdown, DOCX, and formatted copy should work, then verify that each export preserves the requested content and formatting. The issue names jsPDF, markdown-pdf, and the Clipboard API as possible starting points.

Written by the indexing model from the issue text.