fix: remove duplicate Settings menu item

[galz10]

Summary

Fixes duplicate Settings... entries in the macOS app menu by removing the extra Settings scene menu contribution and keeping a single settings command wired to IDX0's inline settings UI.

Details

• Removed the standalone Settings { ... } scene from idx0/App/idx0App.swift.
• Kept CommandGroup(replacing: .appSettings) with the existing Settings... action and keyboard shortcut (.openSettings).
• This ensures only one Settings... item appears in the app menu while preserving the existing inline settings flow.

Related Issues

Closes #15

How to Validate

1. Build the app:
   • xcodebuild -project idx0.xcodeproj -scheme idx0 -destination 'platform=macOS' build
   • Expected: ** BUILD SUCCEEDED **
2. Launch IDX0.
3. Open the app menu (IDX0 in the menu bar).
4. Verify there is only one Settings... item.
5. Click Settings... and verify the inline settings view opens as before.

Pre-Merge Checklist

• Builds successfully (xcodebuild / Xcode)
• Added/updated tests (if needed)
• Updated relevant documentation and README (if needed)
• Noted breaking changes (if any)
• Tested on macOS