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/editor.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "core.h"
+#include "themes/theme.h"
 #include <QWidget>
 #include <QSet>
 #include <QPoint>
@@ -47,6 +48,7 @@ public:
     void setCommandRowText(const QString& line);
     void setEditorFont(const QString& fontName);
     static void setGlobalFontName(const QString& fontName);
+    void applyTheme(const Theme& theme);
 
     // Custom type names (struct types from the tree) shown in type picker + lexer GlobalClass coloring
     QString textWithMargins() const;