Export selected Figma root for Studio imports

[chubes4]

Summary

• export the current Figma selection as the Studio import root
• fall back to the current page when nothing is selected
• update plugin UI/docs to describe selected-frame/current-page import behavior

Testing

• npm run check --prefix plugins/figma-to-wordpress-studio
• npm test --prefix plugins/figma-to-wordpress-studio
• npm run build --prefix plugins/figma-to-wordpress-studio

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Diagnosing the local Figma-to-Studio import result, updating the Figma export root behavior, and running verification commands.