feat(plot): page setup manager — named setups, layouts, <none>/<previous>

Restructure the plot dialog into a style-manager-style master/detail: a
left list of page setups plus a right settings editor. The list shows
<none> (defaults + PDF) and <previous> (the settings the dialog opened
with), then the drawing's layouts (*Model*, *Layout1*, …) and its
standalone named page setups.

- Named page setups live in the ACAD_PLOTSETTINGS dictionary
  (src/scene/page_setup.rs: names/get/save/delete/rename, creating the
  dictionary if absent); DWG round-trip verified.
- Toolbar New / Copy / Delete | Set current / Preview / Print. Selecting
  a row loads it into the editor; Set current writes to the active
  layout. Copy duplicates a layout or a setup into a new named setup;
  Delete/Rename apply only to named setups.
- Scene::plot_settings_for(name) generalizes effective_plot_settings so
  any layout's settings can be loaded; on open the settings are
  snapshotted for <previous>.
- Default selection: model space → <none>; a layout → its own setup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1466,7 +1466,7 @@ impl OpenCADStudio {
             PluginManager => (520, 460),
             UpdateNotice => (560, 460),
             Layers => (900, 360),
-            Plot => (660, 540),
+            Plot => (760, 540),
             LayoutManager => (640, 320),
             Plotstyle => (780, 540),
             TextStyle => (860, 480),

src/app/view/modal.rs

@@ -48,7 +48,7 @@ impl OpenCADStudio {
                 sized(tab.layers.view_window(), 900, 360)
             }
             super::super::ModalKind::Plot => {
-                sized(crate::ui::window::plot::view_window(&self.plot_dialog), 660, 540)
+                sized(crate::ui::window::plot::view_window(&self.plot_dialog), 760, 540)
             }
             super::super::ModalKind::LayoutManager => {
                 let i = self.active_tab;