refactor: centralize hardcoded color values into Colors namespace

Add src/util/colors.hpp with named constants for all QColor values
(brand accents, badge colors, text shades, surface backgrounds) and
replace scattered QColor constructor calls across 7 source files.
Stylesheet string colors are intentionally left inline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/model/tracklistmodel.cpp

@@ -1,4 +1,5 @@
 #include "tracklistmodel.hpp"
+#include "../util/colors.hpp"
 
 #include <QJsonValue>
 #include <QColor>
@@ -255,7 +256,7 @@ QVariant TrackListModel::data(const QModelIndex &index, int role) const
             QFont f; f.setBold(true); return f;
         }
         if (role == Qt::ForegroundRole)
-            return QColor(0xFF, 0xB2, 0x32);
+            return Colors::QobuzOrange;
         return {};
     }
 
@@ -278,8 +279,8 @@ QVariant TrackListModel::data(const QModelIndex &index, int role) const
     }
 
     if (role == Qt::ForegroundRole) {
-        if (!t.streamable) return QColor(0x55, 0x55, 0x55);
-        if (isPlaying)     return QColor(0xFF, 0xB2, 0x32); // Qobuz orange
+        if (!t.streamable) return Colors::DisabledText;
+        if (isPlaying)     return Colors::QobuzOrange;
     }
 
     if (role == Qt::DecorationRole && index.column() == ColNumber && isPlaying) {