Delete the sketch plane dropdown; the viewport decides

The plane combo is gone. A sketch takes its plane from what is picked in
the viewport: a face on a solid, or one of the reference-plane ghosts,
clicked in 3D. The card is now a single line of instruction instead of a
control.

The combo had become worse than redundant. Once a picked face could be
the plane it displayed a row that CONTRADICTED the actual target — it
still said XY while the sketch went onto the face — so the one place a
user could look to confirm where they were drawing was the one place
guaranteed to be wrong.

What replaces it is state, not UI: m_ref_plane records which reference
plane was last clicked in 3D (0/1/2 = XY/XZ/YZ, >=3 indexes the datums)
and ref_plane_name() turns it into text for the on-geometry hint. Clicking
a ghost plane while a session is live re-planes it immediately, which the
combo's own handler used to do; that behaviour is kept, just driven from
the geometry instead of the widget. A plane click also drops a stale face
pick, so last pick wins in both directions.

populate_plane_choices() stays — seven other pickers use it (Plane base,
Axis A/B, Helix, Project, Mirror, Cut). Those are the next candidates,
tracked on snaporca-e1p; this commit only removes the one that had become
actively misleading.

Also: tessellation now matches Orca's OWN STEP importer, linear deflection
0.003 instead of 0.01 (Format/STEP.hpp default; angular was already 0.5
rad and unchanged). The Design viewport was never using a different
rendering technique — it hosts a real GLCanvas3D, builds a real
Model/ModelVolume and goes through the same reload/GLVolume path and the
same shaders as Prepare and Preview. What differed was the mesh handed to
it: 3.3x coarser than anything else in the application, which is why a
curved face read as faceted beside an imported part. Suite unaffected at
154 cases / 2125 assertions, so nothing depended on the old density.

Verified on :10: the card shows no dropdown, one click on a face then a
sketch tool still reports "on the picked face", and the circle is drawn in
that face's plane (artifacts/shots/h3a2-02-sketch.png, h3a2-03-drawn.png).

snaporca-e1p, snaporca-3a2.

src/libslic3r/CadDocument.hpp

@@ -424,7 +424,13 @@ public:
     // world 0). Not serialized — the GUI re-applies it from the live bed on every tab show.
     Vec3d modeling_origin{Vec3d::Zero()};
 
-    double linear_deflection{0.01};
+    // Tessellation quality, matched to Orca's OWN STEP importer (Format/STEP.hpp defaults:
+    // linear 0.003, angular 0.5 rad) so a body modelled here reaches the screen at the same
+    // density as the identical body imported through Prepare. It was 0.01 linear — 3.3x coarser
+    // than anything else in the app, which is why curved faces read as faceted next to an
+    // imported part. Angular already matched. Same BRepMesh_IncrementalMesh call, same GLVolume
+    // path, same shaders: the renderer was never the difference, the mesh fed to it was.
+    double linear_deflection{0.003};
     double angular_deflection{0.5};
 
     int  add_sketch(SketchShape shape, const SketchPlane& plane,