Design tab: Text dialog with font, height and live outline; offer header

- Text opens a dialog instead of a bare text entry: any installed font
  (bold, italic), the height in mm, and a live outline of exactly what
  will be inserted with its size. Enter inserts, Esc cancels; the last
  font and height are remembered. text_to_regions gains an overload
  taking a loaded font.
- The offer menu opens with a greyed title naming what the rows act on
  ("Flat face 4 of Body 2", "Sketch line", "Nothing selected").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QK4VgguuCAk2hZLWgcjJb9

src/slic3r/GUI/CAD/DesignPanel.hpp

@@ -755,6 +755,7 @@ private:
     // monitor, and the menu would map there — detached from the geometry it is about.
     wxPoint offer_anchor() const;
     int  offer_selection_kind() const;          // an OfferSel, as int to keep the header light
+    wxString offer_header(int kind) const;      // the offer menu's title line for that selection
     // Does the SKETCH half of the map apply? A mode question, not a session one: begin_sketch
     // does not run until the first tool is armed, so between "press Sketch" and "pick a tool"
     // is_sketching() is still false — precisely when the drawing tools must be on offer. The