Preserve PPTX background images and dominant text styles during import - #162

Summary

• Import slide and layout background images as editable, full-page image layers.
• Preserve background images when copying slides through the clipboard, including blob-backed assets.
• Use the dominant text run to determine imported styling.
• Improve text contrast handling when a slide uses an image background.
• Add PPTX import and clipboard regression coverage.

Testing

• Added unit coverage for image backgrounds, dominant text styling, and clipboard asset transfer.
• Added an opt-in regression test for the browser AI presentation sample.
• Not run (not requested)