fix: frameless window - all corners rounded + resize fixed

- CentralWidget with bottom-left/bottom-right radius 8px
- Event filter on central widget to forward mouse events for resize
- Consolidated windowFlags (Qt::Window | Frameless | MinMax | Close)
- setMouseTracking(true) for cursor updates
- Removed WA_TranslucentBackground causing black corners
- Status bar fully removed (setStatusBar(nullptr))

src/ui/mainmainwindow.h

@@ -42,6 +42,7 @@ protected:
     void mouseReleaseEvent(QMouseEvent *event) override;
     bool nativeEvent(const QByteArray &eventType, void *message, qintptr *result) override;
     void changeEvent(QEvent *event) override;
+    bool eventFilter(QObject *watched, QEvent *event) override;
 
 private slots:
     void onNavChanged(int index);
@@ -144,6 +145,4 @@ private:
 
     int m_currentFolderId;
     int m_currentMailId;
-
-    QProgressBar *m_progressBar;
 };