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/update/mod.rs

@@ -5442,6 +5442,8 @@ impl OpenCADStudio {
                     header.angle_base = degrees.to_radians();
                 }
                 header.insertion_units = state.insertion_units;
+                self.tabs[i].scene.sync_annotation_scale_from_header();
+                self.tabs[i].scene.invalidate_annotation_dependencies();
                 self.tabs[i].dirty = true;
                 self.tabs[i].scene.bump_geometry();
                 self.refresh_properties();
@@ -6020,6 +6022,8 @@ impl OpenCADStudio {
                 // menu having done nothing. (#668)
                 self.push_undo_snapshot(i, "UNITS");
                 self.tabs[i].scene.document.header.insertion_units = code;
+                self.tabs[i].scene.sync_annotation_scale_from_header();
+                self.tabs[i].scene.invalidate_annotation_dependencies();
                 self.tabs[i].dirty = true;
                 let label = crate::modules::draw::units::label(code);
                 self.command_line.push_output(