fix(properties): close color picker when a panel dropdown opens (#235)

The Properties panel color picker is a custom bool-driven dropdown, while
Layer/Linetype/Lineweight are combo_boxes. Combo_boxes close each other on
blur, but nothing closed the color picker — so opening the Layer dropdown
while the color picker was open left both showing and overlapping.

Give every panel combo_box an on_open handler that collapses the color
picker, so at most one panel dropdown is open at a time.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update/mod.rs

@@ -2123,6 +2123,13 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::PropColorPickerClose => {
+                let i = self.active_tab;
+                self.tabs[i].properties.color_picker_open = false;
+                self.tabs[i].properties.color_palette_open = false;
+                Task::none()
+            }
+
             Message::PropColorPaletteToggle => {
                 self.tabs[self.active_tab].properties.color_palette_open =
                     !self.tabs[self.active_tab].properties.color_palette_open;