fix(plot): write page setups in the file format's canonical form

One writer (plot_settings_from_dialog) now serves both the layout's embedded
settings and named page setups, so the two can no longer drift apart:

- media are stored as the portrait sheet with the orientation in the
  rotation code (0/90/180/270; upside-down adds a half turn) instead of
  swapped width/height with rotation limited to 0/180
- standard scales are written by their code (1:1 … 100:1) with matching
  numerator/denominator; other scales as a readable custom ratio in the
  layout's own paper unit, converting from the drawing's scale-family unit
  (a metre drawing plotting "1:100" stores 10 mm = 1 unit, an inch layout
  stores 1 in = 2540 units); reading converts back and keeps a ratio the
  scale list has no name for instead of snapping to 1:1
- the layout's paper unit and every flag the dialog does not edit survive a
  round trip untouched; layout limits follow the paper-space unit
- new drawings, new layouts, CLEAR and the automation "new" op share one
  default page setup (canonical A4 name, rotation 90, none_device, 1:1
  layout plot) through Scene::reset_to_new_drawing

Scene gains scale_family_unit_mm, scale_ratio and a scale-name ratio parser
(ratios and architectural names). Round-trip tests cover orientation,
standard/custom/fit scales, unit conversion, named-setup parity, preserved
flags and the default layout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -1508,8 +1508,7 @@ impl OpenCADStudio {
             Message::ClearScene => {
                 let i = self.active_tab;
                 self.push_undo_snapshot(i, "CLEAR");
-                self.tabs[i].scene.clear();
-                crate::io::linetypes::populate_document(&mut self.tabs[i].scene.document);
+                self.tabs[i].scene.reset_to_new_drawing();
                 self.tabs[i].properties = PropertiesPanel::empty();
                 let doc_layers = self.tabs[i].scene.document.layers.clone();
                 let vp_info = self.tabs[i].scene.viewport_list();