Design: Measure-style dimension labels (+ projection fix), New Design, tree auto-fit, i18n pin, UX

Sketch dimension labels — now identical to the Prepare/Preview Measure gizmo:
- draw_text repurposed to draw_dim_label: white ImGui text in a translucent-white box,
  mirroring GLGizmoMeasure::render_dimensioning exactly (push_common_window_style sets the
  text colour, BringWindowToDisplayFront, imgui_internal.h).
- ROOT-CAUSE FIX: world_to_screen_px multiplied two Eigen Transform3d objects
  ((proj * view).matrix()); a projection is not affine so Eigen mangled it -> garbage screen
  coords, so labels never appeared. Now proj.matrix() * view.matrix() like Measure. (That
  helper was previously [[maybe_unused]] dead code, never exercised.)
- Leaders: offset clear of the sketch line (no longer coincident with the geometry), single
  point-to-point dimension line + arrows, neutral colour, width 0.6 -> 0.2.
- dim_text appends mm/in on linear dims (angles keep the degree sign).

New Design + delete:
- New "New Design" button wipes the whole document (confirm dialog) — the clear-all the
  per-row Delete can't give. CadDocument::clear() now also clears bodies + display_body_meshes
  (it left them stale, so solids lingered after a clear).
- on_delete_feature: a Body-row selection now shows a helpful hint (bodies are recomputed
  results with no directly-removable feature) instead of silently doing nothing.

Feature tree: auto-fits its content (refresh_tree clamps height 1..9 rows, scrolls past),
instead of a fixed 140px block.

i18n (Design tab pinned English, per the UX contract):
- Restore the lost #undef _L / #define _L(s) wxString::FromUTF8(s) override atop DesignPanel.cpp;
  wrap all ~54 dropdown options in _L so the single lever governs them. feature_type_name left
  untranslated (machine-facing MCP JSON).

UX: per-card Value Confirm/Cancel buttons removed — the single ribbon action bar owns value
confirm/cancel via an m_value_cont guard in tool_confirm/tool_cancel. "needs a body" status
messages unified.

Both forks; DesignSketchTool.{cpp,hpp} + CadDocument.cpp byte-identical across forks. Built
clean; New Design / feature-delete / rotation / tree auto-fit live-verified on :10.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -111,6 +111,7 @@ private:
 
     // Feature-tree editing (Onshape-style): act on the selected tree row.
     void on_delete_feature();
+    void on_new_design();
     void on_move_feature(int delta);   // -1 = up, +1 = down
     void on_toggle_visibility();       // show/hide the selected feature (CadFeature::enabled)