Add theme system with Reclass Dark and Warm built-in themes

Replaces ~40 hardcoded color values with 27 semantic color roles.
Adds ThemeManager singleton, theme editor dialog, View > Theme menu,
JSON persistence for user themes, and fixes inline edit selection
color from blue #264f78 to #2b2b2b.

src/themes/thememanager.h

@@ -0,0 +1,38 @@
+#pragma once
+#include "theme.h"
+#include <QObject>
+#include <QVector>
+
+namespace rcx {
+
+class ThemeManager : public QObject {
+    Q_OBJECT
+public:
+    static ThemeManager& instance();
+
+    QVector<Theme> themes() const;
+    int currentIndex() const { return m_currentIdx; }
+    const Theme& current() const;
+
+    void setCurrent(int index);
+    void addTheme(const Theme& theme);
+    void updateTheme(int index, const Theme& theme);
+    void removeTheme(int index);
+
+    void loadUserThemes();
+    void saveUserThemes() const;
+
+signals:
+    void themeChanged(const rcx::Theme& theme);
+
+private:
+    ThemeManager();
+    QVector<Theme> m_builtIn;
+    QVector<Theme> m_user;
+    int m_currentIdx = 0;
+
+    int builtInCount() const { return m_builtIn.size(); }
+    QString themesDir() const;
+};
+
+} // namespace rcx