feat: My Tasks widget, event countdown redesign, completed section

- My Tasks widget: today's tasks with interactive check-off (ToggleTaskIntent)
  and a "+" that opens the app's add-task sheet; writes preserve all task
  fields and sync via the App Group.
- Event Countdown: AZURE/AWS-style header (accent name + countdown label +
  percent); tap the label to cycle days → weeks → time remaining.
- Today: collapsible "Completed" archive section below Upcoming.

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

KisaniCal/Models/TaskItem.swift

@@ -200,6 +200,12 @@ class TaskViewModel: ObservableObject {
             .filter { $0.isComplete && ($0.completedAt ?? .distantPast) >= startOfDay }
             .sorted { ($0.completedAt ?? .distantPast) > ($1.completedAt ?? .distantPast) }
     }
+    /// All completed tasks, most-recent first (for the Completed archive section).
+    var completedTasks: [TaskItem] {
+        tasks
+            .filter { $0.isComplete }
+            .sorted { ($0.completedAt ?? .distantPast) > ($1.completedAt ?? .distantPast) }
+    }
 
     func toggle(_ task: TaskItem) {
         guard let idx = tasks.firstIndex(where: { $0.id == task.id }) else { return }