feat: Add feature browser dock widget with document synchronization

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

src/Document.h

@@ -19,6 +19,10 @@ public:
     bool save(const QString& path) const;
     bool load(const QString& path);
 
+signals:
+    void featureAdded(Feature* feature);
+    void cleared();
+
 private:
     QList<Feature*> m_features;
 };