fix(ui): adaptive text color for breadcrumb pills (#200)

Use WCAG relative luminance to choose dark or light text color based
on pill background brightness. Bright backgrounds (Green, Yellow,
Mauve, Teal, Peach) now get dark text (#1e1e2e) instead of the
previous fixed light text (#cdd6f4) that caused poor contrast.

Closes #200

src/Gui/BreadcrumbToolBar.cpp

@@ -42,7 +42,13 @@ using namespace Gui;
 static QString chipStyle(const QString& bgHex)
 {
     QColor bg(bgHex);
-    QColor textColor(QStringLiteral("#cdd6f4"));  // Catppuccin Text
+
+    // Choose text color based on background luminance (WCAG relative luminance)
+    // Dark text on bright backgrounds, light text on dark backgrounds
+    double luminance = 0.2126 * bg.redF() + 0.7152 * bg.greenF() + 0.0722 * bg.blueF();
+    QColor textColor(luminance > 0.5
+                     ? QStringLiteral("#1e1e2e")   // Catppuccin Base (dark)
+                     : QStringLiteral("#cdd6f4"));  // Catppuccin Text (light)
 
     return QStringLiteral(
                "QToolButton {"