View backgrounds: image colours as stored, viewport style name, adjust automation

- The background image texture is not an sRGB view: the image is drawn with
  its own colours, as the solid and gradient colours are.
- An entered paper-space viewport shows its named visual style (Realistic,
  ...) in the render-mode picker, and a mode picked there is recorded as the
  viewport's visual style.
- The view_background automation action takes offset=x,y and scale=x,y.

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

src/app/commands/pdf_dialogs.rs

@@ -582,6 +582,8 @@ impl OpenCADStudio {
             | PdfDialogMsg::BgBrowse
             | PdfDialogMsg::BgPosition(_)
             | PdfDialogMsg::BgAspect(_)
+            | PdfDialogMsg::BgOffset(_)
+            | PdfDialogMsg::BgScale(_)
             | PdfDialogMsg::BgApply) => return self.update_view_background(message),
             message @ (PdfDialogMsg::SecPerimeter(_)
             | PdfDialogMsg::SecMaxPoints(_)

src/app/commands/view_presets.rs

@@ -507,6 +507,8 @@ impl OpenCADStudio {
             M::BgImage(value) => state.image = value,
             M::BgPosition(value) => state.position = value,
             M::BgAspect(on) => state.aspect = on,
+            M::BgOffset(value) => state.offset = value,
+            M::BgScale(value) => state.scale = value,
             M::BgColor(slot, rgb) => match slot {
                 0 => state.solid = rgb,
                 1 => state.top = rgb,