fix(analytics): mark WorkoutSnapshot.init(vm:) @MainActor

Build error (Xcode, Swift 6 strict concurrency): WorkoutSnapshot.init(vm:) read
history/workoutDates/schedule/missedDates from @MainActor WorkoutViewModel in a
nonisolated init. The init is only ever called from AnalyticsService.reconcile
(@MainActor) — mark it @MainActor to match its actual call site.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal/Analytics/AnalyticsService.swift

@@ -36,7 +36,7 @@ final class AnalyticsService: ObservableObject {
         var healthByDate: [String: RawHealthDay] = [:]   // HealthKit reference
 
         init() {}
-        init(vm: WorkoutViewModel) {
+        @MainActor init(vm: WorkoutViewModel) {
             self.logByDate = Dictionary(vm.history.map { ($0.date, $0) }, uniquingKeysWith: { a, _ in a })
             self.workoutDates = Set(vm.workoutDates)
             self.scheduledWeekdays = Set(vm.schedule.keys)