Open Markdown documents from HTTPS URLs

[spaquet]

Request

Allow a user to open a hosted Markdown document by pasting its HTTPS URL into Markify. This should work for direct Markdown URLs and public GitHub file URLs such as a repository README.md.

Expected behavior

• Add an Open URL action that fetches the Markdown source and opens it in the editor.
• Accept HTTPS URLs only. Reject http:// URLs and any redirect that downgrades the request to HTTP.
• For GitHub file pages, fetch the raw Markdown source rather than the GitHub HTML page.
• Handle network, HTTP, and non-Markdown responses with a clear error; never open an HTML error page as Markdown.
• Treat the fetched document as a local unsaved copy so edits require an explicit local save. Keep the source URL available for attribution and for resolving relative links and images.

Scope

Public documents first. Protected documents and optional saved credentials are tracked in the dependent authentication issue.