Add visionOS (Apple Vision Pro) app target

[wdvr]

Summary

• Add new "trivit Vision" target for visionOS (Apple Vision Pro)
• SwiftUI views adapted for spatial computing: 3D counter cards, tally marks, add trivit view
• Includes SwiftData model, color theme, asset catalog, and entitlements
• Updates Xcode project with new target configuration

Test plan

• Build the visionOS target in Xcode with visionOS simulator
• Verify counter cards render correctly in spatial environment
• Test adding, incrementing, and deleting trivits
• Confirm tally marks display properly
• Verify iOS target still builds and works correctly

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