View backgrounds: reference blend, image adjust, layouts, cleanup, style name

- Gradients: the middle colour sits (horizon - 0.5) viewport widths from the
  centre and blends over height viewport heights each side; two colours
  blend over a viewport height each side.
- Image backgrounds: offset in device pixels (right, down), negative scale
  mirrors, fitted with aspect ratio covers the viewport; the dialog keeps an
  image's offset and scale.
- Layouts: a model-space view restored on a sheet asks for the viewport,
  enters it and gives it the view's camera, background and visual style;
  inside a viewport the view goes there; a paper-space view's background is
  kept on the sheet viewport but not drawn; floating viewports draw their
  background.
- Background objects no view or viewport uses any more are erased; a view
  without a background restored where the viewport already is leaves the
  viewport's background.
- The viewport's named visual style (Realistic, ...) shows by name and the
  render-mode picker records its style on the viewport record.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/viewport.rs

@@ -1091,6 +1091,10 @@ impl OpenCADStudio {
             return Task::none();
         }
         self.apply_render_mode(i, mode, label);
+        // The viewport record carries the style the mode is saved as.
+        self.tabs[i]
+            .scene
+            .set_active_visual_style_named(crate::modules::view::visual_style::style_name_for(mode));
         self.command_line
             .push_output(crate::tf!("Visual style: {label}").as_ref());
         // The point cloud move gizmo follows the visual style.