feat: Add feature browser dock widget with document synchronization

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

src/Document.cpp

@@ -19,12 +19,14 @@ Document::~Document()
 void Document::addFeature(Feature* feature)
 {
     m_features.append(feature);
+    emit featureAdded(feature);
 }
 
 void Document::clear()
 {
     qDeleteAll(m_features);
     m_features.clear();
+    emit cleared();
 }
 
 bool Document::save(const QString& path) const