optimize tiled background image rendering

[RaajeevChandran]

Summary

• The previous tiled background implementation created a grid of individual SwiftUI Image views using nested ForEach loops.
• For large windows with small tile images, this could generate hundreds of views just for a static background.
• Replaced it with an NSViewRepresentable that uses NSColor(patternImage:) delegates tiling to Core Graphics which renders it as a single pattern fill regardless of window or tile size.

Changes

• Behavior change
• UI change (screenshots below)
• Refactor / chore
• Tests
• Docs

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+