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/Views/TodayView.swift

@@ -1096,7 +1096,7 @@ struct AddTaskSheet: View {
 
     private func submit() {
         guard canAdd else { return }
-        taskVM.addTask(title: cleanTitle, dueDate: parsed.date,
+        taskVM.addTask(title: cleanTitle, dueDate: parsed.date, hasTime: parsed.hasTime,
                        quadrant: selectedQuadrant, category: selectedCategory)
         dismiss()
     }