feat(properties): make read-only values selectable text instead of click-copy

Replace the click-to-instantly-copy read-only cell with a non-editable but
selectable text field: the user selects the value (drag / double-click /
Ctrl+A) and copies with Ctrl+C, the normal expected gesture. The field
carries the full un-truncated value, styled flat (no input box or focus
ring) so it still reads as read-only but with a visible selection colour.
Keystrokes route to Noop, so it can be selected/copied but never edited.

Removes the PropCopyValue message + handler and the mouse_area copy hook.

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

src/app/update/mod.rs

@@ -2107,8 +2107,6 @@ 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