feat(properties): collapse the panel to a vertical bar on a narrow window

When the window is too narrow for the full properties panel it now
collapses to a slim vertical "Properties" bar (its label drawn rotated 90°
on a canvas), freeing width for the viewport. Clicking the bar expands the
panel back out alongside the bar; clicking it again re-collapses
(props_expanded state + TogglePropertiesBar). Reuses the rotated-label bar
helper originally built for the Start page.

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

src/app/update/mod.rs

@@ -226,6 +226,11 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::TogglePropertiesBar => {
+                self.props_expanded = !self.props_expanded;
+                Task::none()
+            }
+
             Message::ScrollLayoutTabs(dx) => iced::widget::operation::scroll_by(
                 iced::advanced::widget::Id::new(crate::ui::statusbar::LAYOUT_TABS_SCROLL_ID),
                 iced::widget::scrollable::AbsoluteOffset { x: dx, y: 0.0 },