Design tab: give the Surface drawer its own icon

The Surface drawer used design_extrude, the same face as the Add-material
drawer, so the two buttons were indistinguishable in the feature bar.
design_surface.svg is a draped patch — deliberately unlike design_plane (a flat
parallelogram) and design_extrude (a box with an up-arrow) — with a faint
interior rule so it reads as a skin rather than a solid face. Same visual
language as the other 71: 24x24, no fill, #b6b6b6, stroke-width 0.85, round
caps and joins.

The five surface ENTRIES that also used design_extrude now use it too. That is
not cosmetic tidying: a flyout button's face follows the last-picked entry
(SetBitmap_(icon_names[i])), so changing only the drawer's default icon would
have been undone the moment the user picked anything. Surface Loft keeps
design_loft, which already suits it.

The six rows still share one glyph between them, so they are told apart by
label alone inside the flyout. Per-entry icons belong with snaporca-vrg
(Draft/Shell reusing design_dressup), not here.

Compiles clean; kernel untouched. Icon confirmed legible on hardware.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

resources/images/design_surface.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#b6b6b6" stroke-width="0.85" stroke-linecap="round" stroke-linejoin="round"><path d="M3 14C7 10 11 18 15 14L21 9C17 13 13 5 9 9Z"/><path d="M6 11.5 12 6.5" stroke-opacity="0.55"/></svg>

src/slic3r/GUI/DesignPanel.cpp

@@ -628,8 +628,10 @@ DesignPanel::DesignPanel(wxWindow* parent)
         fadd("pattern", b_pattern);
 
         // Surface: sheet-body tools (extrude / revolve / loft / fill / offset / thicken)
-        feat_dropdown("surface", "design_extrude", _L("Surface (extrude / revolve / loft / fill / offset / thicken)"), {
-            {"design_extrude", _L("Surface Extrude"), _L("Extrude a sketch into a sheet body (no end caps)"),
+        // design_surface, not design_extrude: sharing a face with the Add-material drawer made
+        // the two buttons indistinguishable in the bar.
+        feat_dropdown("surface", "design_surface", _L("Surface (extrude / revolve / loft / fill / offset / thicken)"), {
+            {"design_surface", _L("Surface Extrude"), _L("Extrude a sketch into a sheet body (no end caps)"),
              [this] {
                 m_surf_extrude_sketch_ref = resolve_extrude_sketch();
                 if (m_surf_extrude_sketch_ref < 0) {
@@ -640,7 +642,7 @@ DesignPanel::DesignPanel(wxWindow* parent)
                 }
                 open_tool(Tool::SurfaceExtrude);
              }, SHIFT('G')},
-            {"design_extrude", _L("Surface Revolve"), _L("Revolve a sketch profile into a sheet body"),
+            {"design_surface", _L("Surface Revolve"), _L("Revolve a sketch profile into a sheet body"),
              [this] {
                 m_surf_revolve_sketch_ref = resolve_extrude_sketch();
                 if (m_surf_revolve_sketch_ref < 0) {
@@ -665,7 +667,7 @@ DesignPanel::DesignPanel(wxWindow* parent)
                 m_surf_loft_refs.clear();
                 open_tool(Tool::SurfaceLoft);
              }, SHIFT('O')},
-            {"design_extrude", _L("Surface Fill"), _L("Fill a sketch boundary with a smooth face"),
+            {"design_surface", _L("Surface Fill"), _L("Fill a sketch boundary with a smooth face"),
              [this] {
                 m_surf_fill_sketch_ref = resolve_extrude_sketch();
                 if (m_surf_fill_sketch_ref < 0) {
@@ -676,7 +678,7 @@ DesignPanel::DesignPanel(wxWindow* parent)
                 }
                 open_tool(Tool::SurfaceFill);
              }, SHIFT('Q')},
-            {"design_extrude", _L("Surface Offset"), _L("Offset a sheet body's shell by a signed distance"),
+            {"design_surface", _L("Surface Offset"), _L("Offset a sheet body's shell by a signed distance"),
              [this] {
                 populate_sheet_body_choices(m_surf_offset_body);
                 if (m_surf_offset_body->GetCount() == 0) {
@@ -687,7 +689,7 @@ DesignPanel::DesignPanel(wxWindow* parent)
                 }
                 open_tool(Tool::SurfaceOffset);
              }, SHIFT('U')},
-            {"design_extrude", _L("Thicken Surface"), _L("Thicken a sheet body into a solid"),
+            {"design_surface", _L("Thicken Surface"), _L("Thicken a sheet body into a solid"),
              [this] {
                 populate_sheet_body_choices(m_surf_thicken_body);
                 if (m_surf_thicken_body->GetCount() == 0) {