feat: Implement feature graph and JSON document save/load

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/Document.h

@@ -0,0 +1,26 @@
+#ifndef DOCUMENT_H
+#define DOCUMENT_H
+
+#include <QObject>
+#include <QList>
+
+class Feature;
+
+class Document : public QObject
+{
+    Q_OBJECT
+public:
+    explicit Document(QObject *parent = nullptr);
+    ~Document();
+
+    void addFeature(Feature* feature);
+    void clear();
+
+    bool save(const QString& path) const;
+    bool load(const QString& path);
+
+private:
+    QList<Feature*> m_features;
+};
+
+#endif // DOCUMENT_H