Fix macOS markdown file associations

[wargoblin]

Summary

Adds macOS content type declarations for Markpad's Markdown and plain text file associations.

Why

The app already declares .md, .markdown, and .txt extensions, but macOS Launch Services works more reliably when document handlers also declare the corresponding UTI/content types. Without those, Markpad may not reliably appear as a recommended/default app in Finder's Open With flow on some macOS installations.

This also changes the document role from Viewer to Editor, which matches Markpad's actual behavior as both a Markdown viewer and editor.

Validation

• cargo check from src-tauri/
• npm run check (existing warnings only, no errors)
• npm run build (existing warnings only)
• npm run tauri build -- --bundles app
• Verified the generated Markpad.app/Contents/Info.plist includes:
  • LSItemContentTypes = [net.daringfireball.markdown, public.markdown]
  • LSItemContentTypes = [public.plain-text]
  • CFBundleTypeRole = Editor

[Copilot]

Pull request overview

Updates Tauri bundle configuration so macOS Launch Services can more reliably associate Markdown and plain text files with Markpad by declaring the corresponding UTIs/content types, and by marking the app as an editor for those document types.

Changes:

• Add contentTypes (UTIs) for Markdown (net.daringfireball.markdown, public.markdown) and plain text (public.plain-text) file associations.
• Change CFBundleTypeRole mapping by setting file association role from Viewer to Editor for .md/.markdown and .txt.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.