Add live app tint picker

[bbauman1]

Summary

• Add a persistent theme picker with 13 common SwiftUI colors to Settings.
• Apply the selected tint at the app root for immediate app-wide updates.
• Add coverage for restoring, validating, and safely defaulting stored theme values.

Validation

• cd Cog && xcodegen generate (GitHub CI)
• Xcode 26 iPhone 17e simulator test suite (GitHub CI)
• Limrun cloud-simulator validation — blocked because this cloud environment has no LIM_API_KEY
• Limrun screenshots — blocked by the same missing credential

GitHub CI passed for commit 4698fc7.