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 <noreply@anthropic.com>

src/app/view/controls.rs

@@ -30,6 +30,7 @@ pub(super) fn viewport_controls<'a>(
     tile_count: usize,
     render_mode_menu_open: bool,
     render_mode_preview: Option<codec::entities::ViewportRenderMode>,
+    style_name: Option<String>,
 ) -> Element<'a, Message> {
     let render_modes: Vec<RenderModeChoice> = crate::modules::view::visual_style::VISUAL_STYLES
         .iter()
@@ -98,7 +99,7 @@ pub(super) fn viewport_controls<'a>(
     // flyout keeps the current style unchanged while hovering; a click commits.
     let picker_head = button(
         row![
-            text(RenderModeChoice(render_mode).to_string()).size(11),
+            text(style_name.unwrap_or_else(|| RenderModeChoice(render_mode).to_string())).size(11),
             crate::ui::icons::themed_arrow_toggle(render_mode_menu_open, 9.0),
         ]
         .spacing(6)
@@ -379,7 +380,7 @@ mod tests {
     #[test]
     fn test_viewport_controls_construction() {
         for style in crate::modules::view::visual_style::VISUAL_STYLES {
-            let _el = viewport_controls(style.mode, true, true, true, 1, false, None);
+            let _el = viewport_controls(style.mode, true, true, true, 1, false, None, None);
             let _el_open = viewport_controls(
                 style.mode,
                 false,
@@ -388,6 +389,7 @@ mod tests {
                 2,
                 true,
                 Some(M::Wireframe3D),
+                None,
             );
         }
     }