iPad layout pass

[ryanabooth]

The app renders on iPad but uses the iPhone layout (single column tab view). Adopt iPad-native patterns.

Scope

• NavigationSplitView for the Tasks / Habits / Goals tabs — sidebar with the list, detail view on the right
• Farm scene — fills the larger canvas; plot tiles bigger; HUD anchored to a corner instead of edge
• Cosmetic shop — two-column grid instead of single column
• Stats screen (once #X lands) — multi-column dashboard
• Settings — sidebar pattern like iOS Settings on iPad

Constraints

• Don't fork the codebase — use size-class checks (@Environment(\\.horizontalSizeClass)) for branching layouts
• iPhone layout remains the default fallback for compact size classes

Acceptance criteria

• All four primary tabs render acceptably on 11" and 13" iPad
• No regressions on iPhone (compact size class)
• No new dependencies