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/viewport.rs

@@ -6031,24 +6031,7 @@ properties={:.1}ms picked={}",
                     if let acadrust::objects::ObjectType::Layout(l) = obj {
                         if l.name == new_name {
                             l.flags = layout_flags;
-                            l.min_limits = (0.0, 0.0);
-                            l.max_limits = (297.0, 210.0);
-                            l.min_extents = (0.0, 0.0, 0.0);
-                            l.max_extents = (297.0, 210.0, 0.0);
-                            l.paper_width = 297.0;
-                            l.paper_height = 210.0;
-                            l.plot_paper_units = 1;
-                            l.plot_scale_numerator = 1.0;
-                            l.plot_scale_denominator = 1.0;
-                            l.plot_scale_type = 16;
-                            l.plot_scale_factor = 1.0;
-                            l.plot_type = 5;
-                            l.plot_flags.use_standard_scale = true;
-                            l.plot_flags.print_lineweights = true;
-                            l.plot_flags.plot_plot_styles = !plot_style.is_empty();
-                            l.plot_flags.show_plot_styles = !plot_style.is_empty();
-                            l.plot_style_sheet = plot_style;
-                            l.paper_size = "ISO_A4_(297.00_x_210.00_MM)".into();
+                            crate::scene::apply_default_page_setup(l, &plot_style);
                             break;
                         }
                     }