Pattern/Cut/Boolean: grey the button when there is no body, and say why

Tommaso reported the array controls as missing. They were not — Shift+N opens a
Pattern card with every control correct — but the report was fair. With no body
the button accepts the click, opens nothing, and writes its refusal somewhere
other than where the click happened. From the user's seat that is
indistinguishable from a dead button, and the icon is one unlabelled glyph among
fourteen, which is how I mis-clicked it into Section view while reproducing this.

A control that cannot act should look like it cannot act, before it is pressed.
The three FEATURE buttons carrying a body-count guard — Pattern and Cut at one
body, Boolean at two — are now greyed below their threshold with a tooltip
naming what is missing.

Only those three. The same guard shape also appears on rows INSIDE the flyouts,
and those stay live: a drawer holds sketch-only entries too, so disabling the
drawer would hide tools that are perfectly usable. The keyboard shortcuts keep
running the guarded action rather than being gated — a key press has no
greyed-out state to see, so the sentence is the only feedback there is.

Re-evaluated in feed_bodies(), before its viewport early-return since this is
about the toolbar and not the canvas, and once after the toolbar is built: an
empty document is the state the bug was reported in and feed_bodies has not run
yet on a fresh tab.

snaporca-o9j. Reviewed and compiled (RC=0), not exercised.

src/slic3r/GUI/DesignPanel.hpp

@@ -272,6 +272,11 @@ private:
     void       update_fillet_gizmo();     // edge-anchored radius arrow (Dressup card)
     void       sync_dressup_target();     // Dressup card: show picked edge vs group, gate the combo
     void       update_hole_gizmo();       // footprint circle + diameter/depth arrows (Hole card)
+    // A FEATURE button whose tool needs bodies it may not have yet. Greyed with an explanatory
+    // tooltip below min_bodies, rather than accepting the click and refusing afterwards.
+    struct BodyGate { wxWindow* btn{nullptr}; int min_bodies{1}; wxString tip_live, tip_gated; };
+    std::vector<BodyGate> m_body_gates;
+    void       update_body_gates();       // re-evaluate them against the current body count
     void       update_thread_gizmo();     // footprint circle + radius/length arrows (Thread card)
     void       update_shell_gizmo();      // inward thickness arrow on the picked face (Shell card)
     void       update_revolve_gizmo();    // angle-arc around the axis (Revolve card)