refactor(ribbon): extract repeated inline color literals into named constants

Consolidates 21 occurrences of identical Color literals scattered across widgets.rs and mod.rs into 9 named constants (COMBO_BG, COMBO_HOVER_BG, COMBO_OPEN_BG, COMBO_BORDER, COMBO_ACTIVE_BORDER, COMBO_ARROW, SWATCH_BORDER, TIP_BG, HIST_INACTIVE_BG). -85 net lines, zero behaviour change.

src/ui/ribbon/mod.rs

@@ -717,12 +717,7 @@ impl Ribbon {
                     .style(move |_: &Theme| container::Style {
                         background: Some(Background::Color(lc)),
                         border: Border {
-                            color: Color {
-                                r: 0.0,
-                                g: 0.0,
-                                b: 0.0,
-                                a: 0.5,
-                            },
+                            color: SWATCH_BORDER,
                             width: 1.0,
                             radius: 1.0.into(),
                         },