Design: Measure-style dimension labels + restore English-only pin + UX consistency

Dimensions (uniform with Prepare/Preview):
- Repurpose DesignSketchTool::draw_text -> new draw_dim_label that renders each
  sketch dimension as the exact Prepare "Measure" gizmo label: white ImGui text
  in a translucent-white box, positioned via the existing world_to_screen_px
  projection inside the active ImGui frame. All 29 label call sites convert with
  no churn; the bespoke Hershey vector font is retired.
- dim_text appends mm/in on linear dims (angles keep the degree sign).
- Placed-dimension leaders simplified to a single point-to-point line + arrows in
  a neutral colour (no extension lines), matching the Measure look.

i18n (Design tab pinned English, per the UX contract):
- Restore the lost "#undef _L / #define _L(s) wxString::FromUTF8(s)" override atop
  DesignPanel.cpp so one lever de-translates the whole tab, ending the half-EN/IT
  state. Wrap all ~54 dropdown options in _L so the single lever governs them.
- feature_type_name left untranslated (it feeds the MCP JSON, machine-facing).

UX consistency:
- Remove the per-card Confirm/Cancel buttons from the Value card; the single ribbon
  action bar now owns value confirm/cancel via an m_value_cont guard in
  tool_confirm/tool_cancel (one confirm surface, per contract).
- Unify the eight divergent "needs a body" status messages to one template.

Both forks; DesignSketchTool.{cpp,hpp} byte-identical across forks. Built clean.

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

src/slic3r/GUI/DesignSketchTool.hpp

@@ -570,6 +570,7 @@ private:
                       double hw, const ColorRGBA& color);
     void draw_text(GLModel& model, const std::string& s, const Vec2d& center,
                    double height, const ColorRGBA& color);                  // GL stroke font
+    void draw_dim_label(const std::string& txt, const Vec2d& plane_center);
 
     // Entity builders: append to m_entities (honoring the construction flag).
     void push_line(const Vec2d& a, const Vec2d& b);
@@ -817,6 +818,7 @@ private:
     GLModel             m_line_model;
     GLModel             m_vertex_model;
     GLModel             m_highlight_model;
+    int                 m_dim_label_seq{0};
     GLModel             m_fill_model;       // translucent face fill for closed regions
     std::vector<DisplaySketch> m_display_sketches;  // committed sketches drawn persistently
     int m_display_pick{-1};        // FEATURE index of the click-selected display sketch (-1 none)