Adds a toggleable honeycomb hexagonal grid overlay (matching the website pattern)
that can be enabled independently of any theme via the Appearance screen. Uses a
cached tiled UIImage approach consistent with the existing grain texture system.
Replaces the destructive refreshID-based theme switching (which destroyed all
NavigationStacks and dismissed sheets) with @Observable AppThemeSource. Color
resolution now happens through Swift's Observation framework, so all views using
Color.appPrimary etc. automatically re-render when the theme changes — no view
identity reset needed.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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