feat: simplify cmd bar keyword, add File menu class/struct/enum, remove Align Members

- Command bar shows static keyword (struct/class/enum) without dropdown or colon
- Right-click keyword in cmd bar for class↔struct conversion (enum blocked)
- File menu: New Class (Ctrl+N), New Struct (Ctrl+T), New Enum (Ctrl+E)
- Project explorer right-click: New Class/Struct/Enum on Project node
- Explorer right-click: Convert to Class/Struct on class/struct items
- Remove Align Members submenu, performRealignment, computeStructAlignment
- Remove screenshot code and screenshot.png

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

src/editor.h

@@ -65,6 +65,7 @@ public:
 signals:
     void marginClicked(int margin, int line, Qt::KeyboardModifiers mods);
     void contextMenuRequested(int line, int nodeIdx, int subLine, QPoint globalPos);
+    void keywordConvertRequested(const QString& newKeyword);
     void nodeClicked(int line, uint64_t nodeId, Qt::KeyboardModifiers mods);
     void inlineEditCommitted(int nodeIdx, int subLine,
                              EditTarget target, const QString& text);