Add task persistence and per-task reminder notifications

Tasks now survive app restarts via UserDefaults JSON. Each task with a
due time fires a notification at that exact time; date-only tasks fire
at 9am on the due date. Completing or deleting a task cancels its
pending notification. ContentView now reschedules on any task change.

KisaniCal/ContentView.swift

@@ -86,11 +86,11 @@ struct ContentView: View {
                 workoutVM.syncWidget()
             }
         }
-        .onChange(of: taskVM.tasks.count) { _ in taskVM.syncWidget() }
-        .onChange(of: workoutVM.doneSets) { _ in
+        .onChange(of: taskVM.tasks) { _ in
+            taskVM.syncWidget()
             NotificationManager.shared.reschedule(workoutVM: workoutVM, taskVM: taskVM)
         }
-        .onChange(of: taskVM.tasks.filter { !$0.isComplete && $0.quadrant == .urgent }.count) { _ in
+        .onChange(of: workoutVM.doneSets) { _ in
             NotificationManager.shared.reschedule(workoutVM: workoutVM, taskVM: taskVM)
         }
     }