Theme system overhaul, UI polish, and VS2022 Dark theme

- Replaced hardcoded theme factories with JSON files + CMake build step
- Shared ThemeFieldMeta table for DRY serialization and editor UI
- Fixed live preview (auto-triggers on color change, no toggle button)
- Fixed duplicate theme entries when editing built-in themes
- Moved title bar from icon to bold "Reclass" text with View > Show Icon toggle
- MDI tabs: 24px height, unicode close button styled like TypeSelectorPopup
- Added VS2022 Dark theme with purple accent colors
- Status bar padding, removed monospace font overrides on tabs/statusbar
- Default startup opens Ball demo + Unnamed hex64 tabs

src/themes/thememanager.h

@@ -31,14 +31,17 @@ signals:
 
 private:
     ThemeManager();
-    QVector<Theme> m_builtIn;
+    QVector<Theme> m_builtIn;          // built-in themes (possibly overridden)
+    QVector<Theme> m_builtInDefaults;  // originals loaded from disk
     QVector<Theme> m_user;
     int m_currentIdx = 0;
 
     int builtInCount() const { return m_builtIn.size(); }
-    QString themesDir() const;
+    void loadBuiltInThemes();
+    QString builtInDir() const;
+    QString userDir() const;
     bool m_previewing = false;
-    Theme m_savedTheme;   // stashed current theme during preview
+    Theme m_savedTheme;
 };
 
 } // namespace rcx