Bars widget: app-driven "Track Countdown" with zero config (KC-20)

- Task long-press menu gains Track/Stop Tracking Countdown, storing the
  tracked event id + tracking date in the App Group (CountdownTracker)
  and reloading widget timelines.
- The widget reads the tracked event directly and bypasses its config:
  Mode A (one-off) = tracking date -> event date; Mode B (recurring) =
  previous occurrence -> next occurrence via the task's recurrence rule.
- WidgetTask decodes isRecurring/recurrenceLabel from the shared JSON.
- With nothing tracked, the configured modes remain the fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

KisaniCal/Views/TaskContextMenu.swift

@@ -1,4 +1,27 @@
 import SwiftUI
+import WidgetKit
+
+/// Tracks one event for the Event Countdown (Bars) widget via App Group storage.
+/// The widget reads these keys directly — no manual widget configuration needed.
+enum CountdownTracker {
+    static let idKey    = "kisani.widget.trackedEvent"
+    static let sinceKey = "kisani.widget.trackedSince"
+
+    static func isTracked(_ task: TaskItem) -> Bool {
+        UserDefaults.kisani.string(forKey: idKey) == task.id.uuidString
+    }
+
+    static func toggle(_ task: TaskItem) {
+        if isTracked(task) {
+            UserDefaults.kisani.removeObject(forKey: idKey)
+            UserDefaults.kisani.removeObject(forKey: sinceKey)
+        } else {
+            UserDefaults.kisani.set(task.id.uuidString, forKey: idKey)
+            UserDefaults.kisani.set(Date(), forKey: sinceKey)   // Mode A anchor
+        }
+        WidgetCenter.shared.reloadAllTimelines()
+    }
+}
 
 /// The unified long-press menu for a task, shared across Today, Matrix, and Calendar.
 /// Pass only the closures a given screen needs; the rest default to no-ops.
@@ -67,6 +90,11 @@ struct TaskMenuItems: View {
             Label("Add to Live Activity", systemImage: "pin.circle")
         }
 
+        Button { CountdownTracker.toggle(task) } label: {
+            Label(CountdownTracker.isTracked(task) ? "Stop Tracking Countdown" : "Track Countdown",
+                  systemImage: "timer")
+        }
+
         if let onEdit {
             Button { onEdit(task) } label: { Label("Edit", systemImage: "pencil") }
         }