refactor: Introduce SketchTool base class and derived tool skeletons

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

src/SketchTool.cpp

@@ -0,0 +1,15 @@
+#include "SketchTool.h"
+#include "ViewportWidget.h"
+
+SketchTool::SketchTool(ViewportWidget* viewport)
+    : QObject(viewport), m_viewport(viewport)
+{
+}
+
+void SketchTool::activate()
+{
+}
+
+void SketchTool::deactivate()
+{
+}