fix: persist user-selected preview view mode after first load (#13625)

fix: apply smart preview defaults per extruder count session

- Track last extruder count (1=single, 2+=multi) instead of boolean flag
- Apply appropriate default (ColorPrint/FeatureType) when count changes
- User selections persist within same extruder count
- Symmetric behavior: both single and multi actively apply defaults
- Delete duplicate dead code block (uncommented TODO scaffolding)

Behavior:
- First slice (any type) → appropriate default
- User changes view → persists on re-slice
- Switch single→single or multi→multi → persists
- Switch single↔multi → appropriate default applies

src/slic3r/GUI/GCodeViewer.hpp

@@ -226,6 +226,7 @@ private:
     std::vector<libvgcode::EViewType> view_type_items;
     std::vector<std::string> view_type_items_str;
     int       m_view_type_sel = 0;
+    int       m_last_extruder_count_default_applied{0};  // 0=unset, 1=single, 2+=multi
     std::vector<EMoveType> options_items;
 
     bool m_legend_visible{ true };