Add app accent theme picker

[bbauman1]

Summary

• Add a persistent theme picker in Settings with six accent colors.
• Apply the selected tint from RootView across the app.

Validation

• cd Cog && xcodegen generate (GitHub Actions)
• iOS simulator test suite (GitHub Actions)
• Limrun cloud-simulator validation — unavailable because this agent environment has no LIM_API_KEY
• Limrun screenshots attached — blocked by the same missing Limrun authentication

CI: https://github.com/bbauman1/cog/actions/runs/29199246968