Fix build: synchronize Request API, fix GmailSynchronizer, and migrate UI to Qt6 Widgets

src/ui/settingsview.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <QWidget>
+#include <QTabWidget>
+#include <QVBoxLayout>
+#include <QHBoxLayout>
+#include <QPushButton>
+#include <QLabel>
+#include <QListWidget>
+#include <QComboBox>
+#include <QCheckBox>
+#include <QRadioButton>
+#include <QButtonGroup>
+#include <QVariant>
+
+class SettingsView : public QWidget {
+    Q_OBJECT
+
+public:
+    explicit SettingsView(QWidget *parent = nullptr);
+    ~SettingsView() override = default;
+
+signals:
+    void accountAddRequested();
+    void accountEditRequested(int accountIndex);
+    void accountDeleteRequested(int accountIndex);
+    void themeChanged(const QString &theme);
+    void settingChanged(const QString &key, const QVariant &value);
+
+private:
+    void setupUI();
+    QWidget* createAccountsTab();
+    QWidget* createGeneralTab();
+    QWidget* createAppearanceTab();
+
+    QTabWidget *m_tabWidget;
+    QListWidget *m_accountList;
+    QComboBox *m_syncIntervalCombo;
+    QButtonGroup *m_themeGroup;
+};