- Convert HealthKitManager to @Observable with async/await, replace deprecated HKWorkout initializer with HKWorkoutBuilder - Replace NavigationView with NavigationStack, use modern toolbar and .alert APIs - Fix fullScreenCover/EmptyView navigation hack with @Environment(\.dismiss) - Add pull-to-refresh, empty state, duration display, date+time pickers, inline validation - Expand activity types from 3 to 20, improve privacy descriptions