feat: add serialization and deserialization for tasks

Introduced methods to serialize and deserialize tasks to enable saving and loading Kanblendar state. This addition facilitates persistence and recovery of task data, ensuring continuity. Adjusted task element creation and placement to maintain task integrity during deserialization.

kanblendar.js

@@ -483,4 +483,33 @@ class Kanblendar {
             section.style.height = `${maxNonTimedHeight}px`;
         });
     }
+
+    serialize() {
+        const tasksArray = Array.from(this.tasks.values()).map(task => ({
+            id: task.id,
+            title: task.title,
+            description: task.description,
+            dueTime: task.dueTime,
+            column: task.column,
+            notifyBefore: task.notifyBefore
+        }));
+        return JSON.stringify(tasksArray);
+    }
+
+    deserialize(serializedData) {
+        const tasksArray = JSON.parse(serializedData);
+        tasksArray.forEach(taskData => {
+            const { id, title, description, dueTime, column, notifyBefore } = taskData;
+            const taskElement = this.createTaskElement(title, description, dueTime, column, notifyBefore);
+            taskElement.id = id; // Restore original ID
+            this.tasks.set(id, { title, description, dueTime, notifyBefore });
+            this.moveTaskToColumn(taskElement, column);
+            if (dueTime) {
+                this.updateTaskLocation(taskElement, dueTime);
+            } else {
+                this.moveTaskToNonTimedSection(taskElement, column);
+            }
+        });
+        this.adjustTimeSlotHeights();
+    }
 }