Design: clicking empty space lets go of the selection, and the status line follows its tab

Two things a click on nothing should already have done.

snaporca-od0. A click that hit no geometry left the solid selection standing. A
rubber band swept over empty space has always cleared it (pick_bodies_in_rectangle),
and the two gestures cannot disagree about the same outcome. The visible cost was in
the escalation that landed last commit: "click the face, click away, click the face
again" arrived as the SECOND click on the same face and took the whole body, when the
click away was the user letting go of it. Now the miss clears and says so.

This gives up something real, deliberately: Thicken / Shell / Draft hold their input
face in the panel's selection, so a stray click on empty canvas with one of those
cards open hands that face back. Their handlers already write the "(pick a solid
face)" placeholder and rebuild the ghost when the selection empties, so the card SAYS
it lost the pick rather than confirming against a face the viewport has stopped
highlighting. An orbit drag never reaches this branch — it exits at the 8px budget —
so panning the view still does not deselect.

snaporca-dlj. The status line is a wxPopupWindow, which is a TOP-LEVEL window: hiding
the Design page does not hide it. Select a face, switch to Prepare, and the chip was
still there reading "selected (whole body) — right-click for what applies to it" on a
tab with no such selection and no such menu. Same cause, second symptom: a status
update arriving while the page is hidden anchored against a client size that is not
the size the page will have, and parked the chip on the tab bar. So: an
IsShownOnScreen guard in place_status_hud, show_status_hud(bool) to take it down and
bring it back with its text intact, driven from the page-changed handler.

Verified on both rigs, not by reasoning about it: face 5 selected -> click bed ->
"Nothing selected", tint gone -> click the same face -> face 5 again, NOT the body ->
click it again with no click away -> whole body, so snaporca-gem is intact. Prepare ->
chip gone; back to Design -> chip returns. KEYTRACE across the round trip shows
shift+S then R still reaching the canvas (ui_mode 0 -> 1, Rectangle armed), which is
the focus theft this popup replaced a wxFrame to avoid.

Fork parity unchanged: DesignPanel.cpp 30, DesignCanvas.cpp 16, headers and
DesignSketchTool.cpp 0. MainFrame.cpp is outside that set and was edited per fork.

src/slic3r/GUI/DesignPanel.hpp

@@ -50,6 +50,7 @@ class DesignPanel : public wxPanel
 public:
     explicit DesignPanel(wxWindow* parent);
     void on_tab_shown();        // re-sync bed to the active printer when the Design tab is activated
+    void on_tab_hidden();       // another tab took over: take the viewport status line down with us
     // Rebuild off the UI thread (progress dialog only if it turns out to be slow), so a feature
     // op on a heavy imported solid does not freeze the window. Returns m_doc.recompute()'s result.
     bool recompute_guarded(const wxString& message);