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/controller.h

@@ -85,6 +85,7 @@ public:
     void removeSplitEditor(RcxEditor* editor);
     QList<RcxEditor*> editors() const { return m_editors; }
 
+    void convertRootKeyword(const QString& newKeyword);
     void changeNodeKind(int nodeIdx, NodeKind newKind);
     void renameNode(int nodeIdx, const QString& newName);
     void insertNode(uint64_t parentId, int offset, NodeKind kind, const QString& name);
@@ -160,7 +161,6 @@ private:
     void connectEditor(RcxEditor* editor);
     void handleMarginClick(RcxEditor* editor, int margin, int line, Qt::KeyboardModifiers mods);
     void updateCommandRow();
-    void performRealignment(uint64_t structId, int targetAlign);
     void switchToSavedSource(int idx);
     void pushSavedSourcesToEditors();
     void showTypePopup(RcxEditor* editor, TypePopupMode mode, int nodeIdx, QPoint globalPos);