A sketch should not look like plate preparation

Three cues, because one is missed. A teal banner across the top of the viewport names the session
("Editing: Sketch N") and where its exits are; the printer bed is muted for the duration, since a
plate grid and a sketch grid are the same visual language and reading one as the other is how a
sketch gets drawn against the wrong reference; and N looks straight down the plane normal at the
current zoom, with the plane's own y axis as up, because no hand-orbit lands exactly square and a
sketch read at an angle is one whose right angles do not look like right angles.

The banner is an INDICATOR. Finish and Cancel stay on the single ribbon action bar — the tab had
three competing confirm surfaces once and that is not being reopened for a strip of colour. It
sits above the canvas rather than floating inside it: a child window over a wxGLCanvas is a native
window on GTK with no reliable stacking over GL, and being unmissable beats being clever.

The bed checkbox stays the stored preference and is restored on leaving the sketch; ticking it
mid-sketch still shows the bed, because that is a deliberate act and this is only a default.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FbJKJAJxxkhDTs9XdZzKA

src/slic3r/GUI/CAD/DesignPanel.hpp

@@ -451,6 +451,13 @@ private:
 
     // Top contextual toolbar (parented to the panel, above the form/viewport row).
     UiMode    m_ui_mode{UiMode::Feature};
+    // Sketch environment banner: a strip across the top of the viewport saying, in words, that
+    // this is a sketch and which one. The mode used to be legible only from the toolbar and the
+    // left card — both of which look like the rest of the app — so a sketch session and plate
+    // preparation were one glance apart. Indicator only: Finish/Cancel stay on the ONE ribbon
+    // action bar (the Design UX contract), and the banner never grows a second pair.
+    wxPanel*      m_sketch_banner{nullptr};
+    wxStaticText* m_sketch_banner_txt{nullptr};
     wxScrolledWindow* m_toolbar{nullptr};   // horizontally scrollable so the action bar stays reachable on narrow windows
     wxSizer*  m_tb_feature{nullptr};
     wxSizer*  m_tb_sketch{nullptr};