Design tab: translatable offer, one vocabulary, reports in the status line

- Offer table: user-facing strings carry the L() marker so xgettext
  extracts them; DesignOffer.hpp, DesignSketchTool.cpp and
  SketchInlineEditor.cpp are listed in localization/i18n/list.txt.
- Offer: model-mode Constrain sits in the same row as the sketch one;
  Interference is wired; Rib shows its R key; a verb that accepts the
  selection but is blocked by the document stays greyed with its reason
  instead of vanishing from the submenu; one refusal wording per verb.
- Extrude infers Join when the profile touches a solid (new
  CadDocument::body_touching_sketch) and on face push/pull; New body in
  free space. Revolve/Sweep/Loft/Boolean use the same result words.
- Interference and volume/area reports go to the status line in mm3/mm2
  instead of modal dialogs; Delete Body no longer asks (it is undoable).
- New feature names match the card header ("Extrude 3"), translated;
  "Coordinate system", "Angle (°)", center/color spelling, translated
  face and length readouts, slot hints say width.
- CAD gizmos in Prepare are selectable only with the CAD feature on; the
  sketch auto-close setting is stored per design.
- Docs: confirm/cancel rules, enabling the feature and MCP in
  design_tab.md; drift-only right-click in interaction-model.md; the
  portability note rewritten to describe the integration as it is.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QK4VgguuCAk2hZLWgcjJb9

src/slic3r/GUI/Plater.cpp

@@ -10074,10 +10074,23 @@ std::vector<size_t> Plater::priv::load_files(const std::vector<fs::path>& input_
     // a user sees after opening a design they spent an hour on — it reads as "your work is gone"
     // when the recipe has in fact just been loaded and the Design tab will rehydrate it. Count a
     // recipe that came from THESE files as geometry.
-    if (tolal_model_count <= 0 && !loaded_cad_recipe && !q->m_exported_file) {
+    //
+    // Only where the Design tab can actually show it, though. With the CAD feature switched off
+    // (or not built) the recipe is still carried through to the next save, but nothing will
+    // display it, so an empty plate needs saying — and saying why.
+    bool cad_can_show = false;
+#ifdef SLIC3R_CAD
+    cad_can_show = wxGetApp().is_enable_cad_feature();
+#endif
+    if (tolal_model_count <= 0 && !q->m_exported_file && (!loaded_cad_recipe || !cad_can_show)) {
         dlg.Hide();
         if (!is_user_cancel) {
-            MessageDialog msg(wxGetApp().mainframe, _L("The file does not contain any geometry data."), _L("Warning"), wxYES | wxICON_WARNING);
+            const wxString text = loaded_cad_recipe
+                ? _L("This project contains a model made in the Design tab and no other geometry. "
+                     "Enable \"CAD feature (experimental)\" in Preferences and restart to see and edit it; "
+                     "it is kept when the project is saved.")
+                : _L("The file does not contain any geometry data.");
+            MessageDialog msg(wxGetApp().mainframe, text, _L("Warning"), wxYES | wxICON_WARNING);
             if (msg.ShowModal() == wxID_YES) {}
         }
     }