Add HTML Page export to SquibView core

[deftio]

Add a complete HTML page export function to the core SquibView library that generates a full HTML page with HEAD elements, DOCTYPE, and all necessary metadata.

Requirements

• Separate from existing copy/getHTML functions
• Include complete HTML document structure with HEAD
• Support for custom CSS injection
• Optional UI button integration
• Works in headless mode

Use Case

Users need to export complete HTML pages that can be saved as standalone files, not just HTML fragments.

Implementation Notes

• Should complement existing getHTML() method
• Consider adding exportHTMLPage() method to main API