Design tab: keyboard shortcuts, plane/axis toggles, section view

- Keyboard shortcuts (Onshape-style, three scoped layers): feature tools on
  Shift+letter, 2D sketch tools on single letters (in-sketch), view/nav on
  single letters (out-of-sketch). Dispatched via the DesignPanel CHAR_HOOK.
- View toggles: P = origin planes, A = world axis triad (render_view_helpers).
- Section view (non-destructive): a single horizontal clip that hides half the
  model to inspect inside, showing only the solid remaining half (no ghost).
  Left-panel "Section View" button or X toggles it; PageUp/PageDown move the
  plane; "Flip Section" button or F shows the opposite half. Never a body/Cut.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignSketchTool.hpp

@@ -98,10 +98,18 @@ public:
     bool has_display() const { return m_active || !m_display_sketches.empty()
                                       || (m_solid_bodies != nullptr && !m_solid_bodies->empty())
                                       || !m_datum_planes.empty()
+                                      || m_show_planes || m_show_axes
                                       || m_ex_active || m_mv_active || m_fl_active
                                       || m_hl_active || m_th_active || m_sh_active
                                       || m_dr_active || m_ct_active || m_dz_active || m_dbp_active; }
 
+    // View helpers: the 3 world origin planes (XY/XZ/YZ) and the world axis triad, each
+    // shown/hidden by a toggle (keys P / A). Off by default so the idle scene stays clean.
+    void set_show_planes(bool s) { m_show_planes = s; }
+    void set_show_axes(bool s)   { m_show_axes = s; }
+    bool toggle_show_planes() { m_show_planes = !m_show_planes; return m_show_planes; }
+    bool toggle_show_axes()   { m_show_axes   = !m_show_axes;   return m_show_axes; }
+
     // Solid topology selection on the committed bodies: clicking a solid cycles
     // whole-solid -> face -> edge (Onshape-style) to target fillet/chamfer/extrude. With
     // multiple bodies the pick resolves WHICH body was hit (per-triangle body id).
@@ -891,6 +899,9 @@ private:
     bool handle_solid_click(GLCanvas3D& canvas, const wxMouseEvent& evt);  // cycle + notify
     void render_solid_highlight();
     void render_datum_planes();           // translucent rectangles for datum/reference planes
+    void render_view_helpers();           // world origin planes + axis triad (P / A toggles)
+    bool m_show_planes{false};
+    bool m_show_axes{false};
     std::vector<SketchPlane> m_datum_planes;
     std::vector<Vec2d>       m_datum_sizes;   // per-plane (u,v) full extent; empty -> default
     GLModel m_solid_face_model;