feat: per-plate iXex mode selection with undo, dark mode icons, and menu fixes

- Add per-plate iXex mode icon to the plate toolbar (normal, hover, dark, dark-hover SVG variants)
- Left-click cycles through available modes; right-click shows a popup menu with all modes as radio items
- Mode changes are recorded in the undo/redo snapshot system
- Fix double context menu: suppress EVT_GLCANVAS_PLATE_RIGHT_CLICK and EVT_GLCANVAS_RIGHT_CLICK when the iXex icon popup was already shown
- Remove ixex_parallel_mode combo from Print Settings > Other > Special mode (superseded by per-plate icon)
- Remove dead code: refresh_ixex_mode_combo(), m_ixex_mode_combo member, related Tab reload hook
- iXex mode persisted in 3MF project files via existing plate metadata serialization

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/slic3r/GUI/GLCanvas3D.cpp

@@ -4648,9 +4648,10 @@ void GLCanvas3D::on_mouse(wxMouseEvent& evt)
             }
 
             //BBS change plate selection
+            bool plate_icon_popup_shown = false;
             if (!m_hover_plate_idxs.empty() && (m_canvas_type == CanvasView3D) && !m_mouse.dragging) {
                 int hover_idx = m_hover_plate_idxs.front();
-                wxGetApp().plater()->select_plate_by_hover_id(hover_idx, true);
+                plate_icon_popup_shown = (wxGetApp().plater()->select_plate_by_hover_id(hover_idx, true) == 1);
                 if (m_hover_volume_idxs.empty())
                     deselect_all();
                 render();
@@ -4670,10 +4671,10 @@ void GLCanvas3D::on_mouse(wxMouseEvent& evt)
 
             if (!m_mouse.ignore_right_up && m_gizmos.get_current_type() == GLGizmosManager::EType::Undefined) {
                 //BBS post right click event
-                if (!m_hover_plate_idxs.empty()) {
+                if (!m_hover_plate_idxs.empty() && !plate_icon_popup_shown) {
                     post_event(RBtnPlateEvent(EVT_GLCANVAS_PLATE_RIGHT_CLICK, { logical_pos, m_hover_plate_idxs.front() }));
                 }
-                else {
+                else if (!plate_icon_popup_shown) {
                     // do not post the event if the user is panning the scene
                     // or if right click was done over the wipe tower
                     bool post_right_click_event = m_hover_volume_idxs.empty() || !m_volumes.volumes[get_first_hover_volume_idx()]->is_wipe_tower;