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/slic3r/GUI/DesignPanel.hpp

@@ -229,6 +229,7 @@ private:
     // Plane pickers: fill a choice with XY/XZ/YZ + the document's datum planes, and
     // map a choice row back to the actual SketchPlane (rows 0-2 base, 3+ datum).
     void        populate_plane_choices(ComboBox* c) const;
+    wxString    ref_plane_name(int row) const;   // "XY" / a datum's name, for the on-geometry hint
     SketchPlane plane_from_choice(int row) const;
     // Where a new sketch goes, resolved from what is SELECTED IN THE VIEWPORT rather than from a
     // list: a picked planar face wins, otherwise the reference plane last clicked in 3D. `what`
@@ -392,7 +393,10 @@ private:
     wxSpinCtrl*       m_sides{nullptr};          // polygon sides
     CheckBox*         m_poly_circ{nullptr};      // polygon circumscribed toggle (Orca teal check)
 
-    ComboBox*         m_draw_plane{nullptr};
+    // Which reference plane a sketch falls back to when no face is picked: 0/1/2 = XY/XZ/YZ,
+    // >=3 indexes resolve_datum_planes(). Set by CLICKING a ghost plane in the viewport — there is
+    // deliberately no dropdown for it. snaporca-e1p.
+    int               m_ref_plane{0};
     ComboBox*         m_shape{nullptr};
     ComboBox*         m_plane{nullptr};
     ComboBox*         m_mode{nullptr};