feat(properties): copy read-only property values to the clipboard

A read-only value cell is now click-to-copy: clicking writes the full,
un-elided value to the system clipboard (copy cursor on hover). One
change in render_ro_row, so it covers every read-only row for every
entity. Adds Message::PropCopyValue routed through iced::clipboard::write.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -2107,6 +2107,8 @@ impl OpenCADStudio {
 
             Message::PropGeomCommit(field) => self.on_prop_geom_commit(field),
 
+            Message::PropCopyValue(value) => iced::clipboard::write(value),
+
             Message::PropAttrInput { tag, value } => {
                 self.tabs[self.active_tab]
                     .properties