Dark mode pass

[ryanabooth]

The app inherits SwiftUI's dark-mode handling for free in most views, but the SpriteKit farm scene and the placeholder tile colors haven't been audited.

Scope

• Run every screen in dark mode in the simulator
• PlotTextureFactory placeholder colors: check contrast against the dark SpriteKit ground; adjust if dim
• Farm scene background: pick light/dark variants or a neutral tint that works in both
• HUD readability: gold counter, quest log button, capacity upgrade button
• Cosmetic shop: art swatch placeholders
• Health bar colors: green/yellow/red palettes that work in dark

Acceptance criteria

• Every screen renders cleanly in dark mode with no "why is this invisible" moments
• No hard-coded Color.white / Color.black that breaks the other mode
• Screenshots of every primary screen in both modes attached to the PR (for the record)