Add FolderDao and MailItemDao, update MailItem model to include folderId, fileId, size, messageId

src/db/dao/folderdao.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "../databasemanager.h"
+#include "../../core/models/folder.h"
+#include <QVector>
+#include <optional>
+
+class FolderDao
+{
+public:
+    static bool insert(const Folder& folder);
+    static bool update(const Folder& folder);
+    static bool remove(int id);
+    static std::optional<Folder> findById(int id);
+    static QVector<Folder> findAll();
+    static QVector<Folder> findByAccountId(int accountId);
+};