feat: custom dock titlebar, resize grip symmetry fix, status bar font sync

- Replace default dock widget titlebar with custom label + themed ✕ close button
- Remove float/popout button from project tree dock
- Fix resize grip corner symmetry (bottom margin 4→0)
- Sync editor font to status bar and dock titlebar at startup
- Add testResizeGripCornerSymmetry test

src/mainwindow.h

@@ -124,6 +124,8 @@ private:
     QDockWidget*        m_workspaceDock  = nullptr;
     QTreeView*          m_workspaceTree  = nullptr;
     QStandardItemModel* m_workspaceModel = nullptr;
+    QLabel*             m_dockTitleLabel = nullptr;
+    QToolButton*        m_dockCloseBtn   = nullptr;
     void createWorkspaceDock();
     void rebuildWorkspaceModel();
     void updateBorderColor(const QColor& color);