Add a minimal macOS Settings menu

[PathGao]

Summary

• reduce the macOS native menu to app-level actions
• add Settings… with the standard Cmd+, shortcut
• route Settings to the currently focused Markpad window

Fixes #281

Validation

• npm ci
• npm run check
• npm test
• cargo test

Local bundle validation

• macOS Bundle: com.alecdotdev.markpad.pr281, version 2.6.14-pr281.local.1
• Built with npm run tauri build in an isolated clone.
• Verified the native menu contains only About, Settings…, Check for Updates…, Services, Hide, and Quit. Settings… opened the existing Settings modal in the focused window.

[PathGao]

@alecdotdev Thank you for transferring the repository to the organization. I checked the current setup: GitHub Stacks is now available in public preview, but I cannot create a native stack yet.

My account currently has read-only access to sftwrdotdev/Markpad, and the heads of my open PRs are branches in the PathGao/Markpad fork. GitHub Stacks requires every branch in the chain to be in the same repository, so cross-fork PRs cannot be linked as a native stack.

If I receive write access to the upstream repository, I can create future dependent branches directly there and use Stacks for the genuine dependency chains. The existing independent PRs will remain separate.

[alecdotdev]

@PathGao perfect, I've granted you write access to the repository

[PathGao]

@alecdotdev Thank you — I appreciate the write access. I will verify which of the existing dependent PRs can be converted into a native stack and report back with the result.