-VIEW sEttings Background dialog and view backgrounds

Color, Gradient and Image at the background prompt open a Background
dialog (None / Solid / Gradient / Image, preview, solid colour, two- or
three-colour gradient with rotation, image file with position and
scale). Apply links the view to a new background object in
ACAD_BACKGROUND; restoring a view gives the model viewport a copy of
it (or none), and saving a view copies the viewport's background.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/view/modal.rs

@@ -35,6 +35,11 @@ impl OpenCADStudio {
             Some(K::PointCloudAttach) => crate::t!("Attach Point Cloud").into_owned(),
             Some(K::PointCloudColorMap) => crate::t!("Point Cloud Color Map").into_owned(),
             Some(K::PcSection) => crate::t!("Extract Section Lines from Point Cloud").into_owned(),
+            Some(K::ViewBackground) => format!(
+                "{} — {}",
+                crate::tr!("view-background", "title"),
+                self.view_background.as_ref().map_or("", |s| s.view.as_str())
+            ),
             Some(K::UnderlayLayers) => crate::t!("Underlay Layers").into_owned(),
             Some(K::PdfImportSettings) => crate::t!("PDF Import Settings").into_owned(),
             Some(K::PdfImportFile) => crate::t!("Import PDF").into_owned(),
@@ -510,6 +515,12 @@ impl OpenCADStudio {
                     crate::ui::window::pdf_dialogs::view_pc_section(state, flow)
                 })
             }
+            super::super::ModalKind::ViewBackground => {
+                let state = self.view_background.as_ref()?;
+                sized_flow(ex, 420, 560, |flow| {
+                    crate::ui::window::pdf_dialogs::view_background(state, flow)
+                })
+            }
             super::super::ModalKind::UnderlayLayers => {
                 let state = self.underlay_layers.as_ref()?;
                 sized_flow(ex, 460, 520, |flow| {