fix(annotative): start new drawings at the unit-aware annotation scale

A new drawing is metric with metre insertion units, but its annotation
multiplier stayed at Scene::new()'s unitless 1 while opened files and
CANNOSCALE convert paper millimetres into drawing units (0.001 at 1:1).
Annotative text and dimensions were drawn 1000x too large until the
first scale change, then shrank to their real size and looked gone,
also after switching back to 1:1.

One helper now derives the multiplier from the header; new drawings,
file open and every INSUNITS change (UNITS, the units popup, DWGUNITS,
SETVAR) go through it.

Fixes #1674

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/styleprops.rs

@@ -3088,6 +3088,10 @@ impl OpenCADStudio {
                                 self.delete_objects = value;
                             }
                             if changed {
+                                if name == "INSUNITS" {
+                                    self.tabs[i].scene.sync_annotation_scale_from_header();
+                                    self.tabs[i].scene.invalidate_annotation_dependencies();
+                                }
                                 if matches!(
                                     name.as_str(),
                                     "ZOOMWHEEL"