feat(gui): Implement Visual DAG with imgui_node_editor

- Add node editor context and config in App.__init__
- Replace tree-based DAG with imgui_node_editor visualization
- Add selection detection for interactive ticket editing
- Add edit panel for selected ticket (view status, target, deps, mark complete, delete)
- Add ui_selected_ticket_id state variable

tests/test_mma_node_editor.py

@@ -0,0 +1,17 @@
+import pytest
+from unittest.mock import MagicMock, patch
+import sys
+
+def test_imgui_node_editor_import():
+    from imgui_bundle import imgui_node_editor as ed
+    assert ed is not None
+    assert hasattr(ed, "begin_node")
+    assert hasattr(ed, "end_node")
+
+def test_app_has_node_editor_attrs():
+    from src.gui_2 import App
+    import inspect
+    source = inspect.getsource(App.__init__)
+    assert 'node_editor_config' in source
+    assert 'node_editor_ctx' in source
+    assert 'ui_selected_ticket_id' in source