feat(ui): use the shared colour selector in properties, ribbon and layers

Replace the bespoke colour pickers with the shared color_selector / color_list
widget so colour selection is identical everywhere:
- left properties panel (selected entities' colour)
- ribbon Home-tab colour dropdown
- layer manager per-layer colour

Each routes its "More…" entry to the standalone Select Color window via a
ColorPickTarget (Properties / Ribbon / Layer). Extracted color_list as the
shared popup content; the colour selector button now fills its container.

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

src/app/mod.rs

@@ -574,6 +574,12 @@ pub enum ColorPickTarget {
     DimStyle(DsField),
     MLeader(&'static str),
     Table(u8, &'static str),
+    /// Selected entities' colour (left properties panel).
+    Properties,
+    /// Current creation colour (ribbon).
+    Ribbon,
+    /// A layer's colour, by panel row index.
+    Layer(usize),
 }
 
 /// Identifies a DimStyle field that can be edited in the dialog.