feat: add Import from Source parser for C/C++ struct definitions

Adds a new "Import from Source..." menu item that opens a QScintilla
editor dialog where users can paste C/C++ struct definitions. The parser
tokenizes and parses the source using recursive descent, supporting
stdint.h types, Windows types (BYTE/DWORD/PVOID/etc), multi-word C
types, pointers, arrays, Vec2/3/4/Mat4x4 detection, unions (first
member), padding fields, typedefs, forward declarations, static_assert
size checks, and auto-detection of comment offset mode vs computed
offsets. Also removes the flaky test_editor cursor shape tests.

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

src/mainwindow.h

@@ -47,6 +47,8 @@ private slots:
     void toggleMcp();
     void setEditorFont(const QString& fontName);
     void exportCpp();
+    void importFromSource();
+    void importReclassXml();
     void showTypeAliasesDialog();
     void editTheme();
     void showOptionsDialog();