feat: iMEX temperature handling and pre-slice warnings

Temperature emission (GCode.cpp, PrintConfig.cpp, ConfigManipulation.cpp):
- Set temperatures for all active tools in layer_change_gcode for iMEX
  parallel modes (primary + secondary carriages)
- Fix filament temperature commands so Layer 1 temperatures only emit on
  the first layer; subsequent layers use normal layer-change temperatures
- Remove stray temperature commands that fired outside the intended context
- Consolidate iMEX temperature handling into the second-layer transition;
  clean up emission logic and naming throughout

Pre-slice warning dialogs (PartPlate.cpp/hpp, Plater.cpp):
- Collect per-plate IMEX warnings before slicing: multi-material conflict,
  bed temperature mismatch between carriages (>5 °C delta), and filament
  type incompatibility
- Show a dismissible Yes/No dialog from both "Slice Plate" and "Slice All"
  actions; No returns to 3D view, Yes proceeds to slice
- Refresh plate thumbnails after the panel switch so previously-generated
  thumbnails are not left black

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

src/slic3r/GUI/ConfigManipulation.cpp

@@ -793,6 +793,9 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig *config, co
     bool has_ironing = (config->opt_enum<IroningType>("ironing_type") != IroningType::NoIroning);
     for (auto el : { "ironing_pattern", "ironing_flow", "ironing_spacing", "ironing_angle", "ironing_inset", "ironing_angle_fixed" })
         toggle_line(el, has_ironing);
+    bool has_rectilinear_ironing = (config->opt_enum<InfillPattern>("ironing_pattern") == InfillPattern::ipRectilinear);
+    for (auto el : {"ironing_angle", "ironing_angle_fixed"})
+        toggle_field(el, has_ironing && has_rectilinear_ironing);
     
     toggle_line("ironing_speed", has_ironing || has_support_ironing);