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/mod.rs

@@ -1435,6 +1435,10 @@ pub enum Message {
     PropAttrCommit(String),
     /// Toggle the inline color picker dropdown open/closed.
     PropColorPickerToggle,
+    /// Collapse the inline color picker dropdown. Fired when another
+    /// properties-panel dropdown (a combo_box) opens, so at most one panel
+    /// dropdown is open at a time and they can't overlap. (#235)
+    PropColorPickerClose,
     /// Toggle the full ACI colour palette expansion.
     PropColorPaletteToggle,
     /// Enter the model-space editing mode inside the given viewport (MSPACE).