feat: annotation scale system (CANNOSCALE, PSLTSCALE, MultiLeader, Leader)

- Scale popup dropdown replaces cycle behavior; paper-space viewport scale
  editable when a viewport is active/selected
- CANNOSCALE persisted via DXF $USERR1 (read on open, write on save)
- PSLTSCALE: linetype dash lengths scaled by viewport anno_scale so dashes
  appear uniform physical size in paper space
- MultiLeader: arrow size, dogleg length, and text height scale with
  anno_scale; leader line paths (world coords) stay fixed
- Leader: arrow size and hookline length scale with anno_scale; also
  corrects world_offset subtraction that was previously deferred to the
  TruckObject::Lines path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/mod.rs

@@ -15,6 +15,7 @@ pub mod plotstyle;
 pub mod properties;
 pub mod ribbon;
 pub mod shortcuts;
+pub mod scale_popup;
 pub mod snap_popup;
 pub mod statusbar;
 pub mod tablestyle;