Fix #40: W/H constrain-proportions link in the Properties panel and Control bar

The Transform panel had the link, but kept it in panel-local egui memory;
the Properties panel's Transform section and the Control bar had none, so
size changes there were never proportional.

The link is now the constrainProportions preference (prefs.get/prefs.set),
one toggle shared by the three places (transform::constrain_link), and every
W/H field passes it to object.setBounds as `proportional`.

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

crates/engine/src/cmd/prefscmds.rs

@@ -320,6 +320,8 @@ pub const PREF_SPECS: &[PrefSpec] = &[
     p!("appendConverted", "File Handling", "Files", "Mark Older Files as [Converted] When Opened", bool),
     // Native saves (`document.save {compress}`)
     p!("useCompression", "File Handling", "Files", "Use Compression", bool),
+    // The W/H link of the Transform panel, the Properties panel and the Control bar
+    p!("constrainProportions", "General", "Transform Panel", "Constrain Width and Height Proportions", bool),
 ];
 
 pub fn spec(key: &str) -> Option<&'static PrefSpec> {