Layouts: first opening follows the page setup; viewports picked by their frame

- A layout opens for the first time when its page setup was never
  initialized (no paper size, no media name) or it was created in this
  session; its page setup is initialized then, and with
  LAYOUTCREATEVIEWPORT it gets its first viewport. A layout never opened
  is saved with an uninitialized page setup and no viewports, so it gets
  its viewport when the file is opened again, here or in the reference.
- Select Viewport for view: a viewport is selected when the pick box
  (PICKBOX pixels at the sheet's zoom) touches its frame; a pick well
  inside or outside selects nothing and the prompt repeats.

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

src/app/update/viewport.rs

@@ -6538,6 +6538,7 @@ properties={:.1}ms picked={}",
                     .as_ref()
                     .map(|style| style.name.clone())
                     .unwrap_or_default();
+                self.tabs[i].scene.new_layout_plot_style = plot_style.clone();
                 for obj in self.tabs[i].scene.document.objects.values_mut() {
                     if let codec::objects::ObjectType::Layout(l) = obj {
                         if l.name == new_name {