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/themeeditor.h

@@ -0,0 +1,29 @@
+#pragma once
+#include "theme.h"
+#include <QDialog>
+#include <QVector>
+#include <QPushButton>
+
+namespace rcx {
+
+class ThemeEditor : public QDialog {
+    Q_OBJECT
+public:
+    explicit ThemeEditor(const Theme& theme, QWidget* parent = nullptr);
+    Theme result() const { return m_theme; }
+
+private:
+    Theme m_theme;
+
+    struct SwatchEntry {
+        const char* label;
+        QColor Theme::*field;
+        QPushButton* button;
+    };
+    QVector<SwatchEntry> m_swatches;
+
+    void updateSwatch(SwatchEntry& entry);
+    void pickColor(SwatchEntry& entry);
+};
+
+} // namespace rcx