SwiftUI performance audit

Docs: https://developer.apple.com/documentation/xcode/understanding-and-improving-swiftui-performance.

• Review code first. Ask for an Instruments SwiftUI trace only when the code review is inconclusive.
• Look for heavy work in body, broad Observation fan-out, unstable identity in lists, GeometryReader and preference chains, and image decoding on the main thread.
• Move heavy work into derived state or the model layer. Use @State for view-owned state, not as a cache. Use equatable() only when equality is cheaper than the subtree.
• Report likely causes, evidence, fix, and how to validate.