Design tab: sheet pickers targeted the wrong body; guard Delete Face's face list

The sheet-only pickers filtered correctly and then threw the filtering away. Their
rows are the SHEET bodies, but GetSelection() was passed straight through as an
index into m_doc.bodies. With a solid at 0 and a sheet at 1 — the normal order,
since you extrude a solid before making a surface — the single row is row 0 but
body 1, so Surface Offset and Thicken Surface targeted the SOLID. The kernel then
refused with "target is not a sheet", which reads as a kernel bug rather than a
picker bug, and the row's own label ("Body 2") disagreed with what was targeted.
Four sites per tool were wrong, including the re-edit path, which compared a body
index against the sheet-only row count and so restored the wrong row.

populate_sheet_body_choices() now carries the real body index in client data, and
two helpers make the row/body distinction hard to get wrong again:
sheet_choice_body() reads it back, select_sheet_choice() finds the row holding a
given body. No caller touches GetSelection()/SetSelection() on these pickers.

This is the third instance of the same index-space confusion in this file, after
the 0-based body labels in the interference report and the Rib sketch picker. The
kernel suite cannot catch any of them: the kernel receives whatever index the GUI
computed, and its own tests pass correct ones.

Delete Face was structurally right — its picker uses the all-bodies populate, so
its indices genuinely match, and accumulation appends with a running list. Two
gaps closed: clicking "Add picked face" with nothing picked was a silent no-op,
indistinguishable from a broken button, and the same face could be added twice,
putting a duplicate id into delete_faces that the defeaturing has no reason to
cope with. Re-adding is now a no-op with a message, not an error.

Both confirmed working on hardware. Kernel untouched: 139 cases / 1960 assertions.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -123,6 +123,10 @@ private:
     // consumed tool body still appears and its saved selection round-trips).
     void populate_body_choices(int as_of_feature = -1);
     void populate_sheet_body_choices(ComboBox* c) const;   // bodies where is_sheet_shape() is true
+    // Rows of a sheet-filtered picker are not body indices; go through these two, never
+    // GetSelection()/SetSelection() directly.
+    static int  sheet_choice_body(ComboBox* c);            // real body index of the current row, or -1
+    static void select_sheet_choice(ComboBox* c, int body);// select the row holding this body index
     // Import rigid 2D art (Text / SVG) as a new Sketch feature carrying
     // imported_regions (no solver entities). on_add_text/on_import_svg gather
     // input; add_imported_sketch builds the feature, refreshes tree + display.