Support dropping markdown files into Writer

[livid]

Summary

Add support for dropping a Markdown file onto the article list for a selected MyPlanetModel.

Expected behavior

• Open a Writer window as a new draft for the selected MyPlanetModel
• Read the dropped Markdown file into the draft body
• If the first non-empty Markdown heading is an H1, use it as the Writer title
• Remove that H1 from the draft body content
• Preserve existing drag-and-drop behavior for non-Markdown files
• If other files are dropped alongside the .md, attach them to the new draft

Notes

• Require a selected MyPlanetModel; otherwise show an error
• Support security-scoped file access for dropped files outside the sandbox
• Create a fresh draft rather than reusing an existing one for the import flow
• Implemented in commit ee8de8a