Fix: Right Edge of G-code Viewer Legend Rows Not Interactive (#11899)

Fix legend hover hit-testing

Fix GCode viewer legend where the eye icon and right edge don’t respond to hover/click unless you click slightly left (works after switching view types unless the vertical slide is used).

src/slic3r/GUI/IMSlider.hpp

@@ -128,6 +128,8 @@ public:
 
     float m_scale = 1.0;
     void set_scale(float scale = 1.0);
+    // ORCA: expose vertical slider width for right-margin alignment in the preview UI.
+    static float vertical_slider_window_width();
     void on_change_color_mode(bool is_dark);
     void set_menu_enable(bool enable = true) { m_menu_enable = enable; }