Add confirmation dialog to deleting a plan

lib/edit_plan.dart

@@ -44,12 +44,41 @@ class _EditPlanPageState extends State<EditPlanPage> {
     super.dispose();
   }
 
+  Future<bool?> _showConfirmationDialog(BuildContext context) {
+    return showDialog<bool>(
+      context: context,
+      barrierDismissible: false,
+      builder: (BuildContext context) {
+        return AlertDialog(
+          title: const Text('Confirm Delete'),
+          content: const Text('Are you sure you want to delete this plan?'),
+          actions: <Widget>[
+            ElevatedButton(
+              child: const Text('Yes'),
+              onPressed: () {
+                Navigator.pop(context, true); // showDialog() returns true
+              },
+            ),
+            ElevatedButton(
+              child: const Text('No'),
+              onPressed: () {
+                Navigator.pop(context, false); // showDialog() returns false
+              },
+            ),
+          ],
+        );
+      },
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     List<Widget> actions = [];
     if (widget.plan.id.present)
       actions.add(IconButton(
           onPressed: () async {
+            bool? confirm = await _showConfirmationDialog(context);
+            if (!confirm!) return;
             await db.plans.deleteOne(widget.plan);
             if (!mounted) return;
             Navigator.pop(context);