Design tab: show/hide the printer bed; give Placement its own toolbar slot

Port of snaporca ca25352d74. Hand-applied rather than cherry-picked: unlike the
Phase B commits, which only touched DesignPanel.{cpp,hpp} and transfer verbatim,
this one reaches into GLCanvas3D and DesignCanvas, where the forks genuinely
differ — mainline passes m_show_world_axes to _render_bed where Snapmaker passes
a local show_axes, and the surrounding code sits ~90 lines further down. git am
refused, correctly; the six edits were applied against mainline's own context and
the DesignPanel half came across as a patch.

Bed toggle: a "Bed" checkbox in the document/view row, on by default, in that row
rather than in a card because a view option must stay reachable with no tool open.
It drives GLCanvas3D::m_show_bed (default true, so Prepare and Preview are
untouched) and gates _render_platelist as well as _render_bed — hiding the bed
while leaving its grid and outline floating would read as a rendering fault.

Bound to wxEVT_TOGGLEBUTTON, not wxEVT_CHECKBOX: Orca's CheckBox derives from
wxBitmapToggleButton, so a wxEVT_CHECKBOX handler never fires.

Also gives the Placement drawer its own "placement" toolbar slot. put("place")
already holds the Place-on-Face button, and put() formats slot item 0 as the
control and later items as its chevron, so sharing the slot bottom-aligned the
drawer's button like a chevron.

196/196 targets, 0 compile errors, orca-slicer links (165 MB). The build script
still exits non-zero at the AppImage bundling step on libpython3.12.so.1.0 —
that is snaporca-96t, packaging only, and does not affect the binary.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -255,6 +255,7 @@ private:
     void      push_polygon_params();
     wxSizer*  m_tb_commit{nullptr};   // far-right Commit to Plate, beside Confirm/Cancel
     wxSizer*  m_tb_doc{nullptr};      // toolbar document/view actions (new, commit, export, section, place)
+    CheckBox* m_show_bed{nullptr};    // view option: draw the printer bed + plate grid, or not
     wxSizer*  m_box_move{nullptr};      // Move/Rotate numeric options (distance, axis, angle)
     wxSizer*  m_box_polygon{nullptr};   // Polygon tool options (sides / circumscribed)
     wxSizer*  m_box_sketch{nullptr};