feat(gui): Implement Takes panel (Phase 4)

- Replaced _render_takes_placeholder with _render_takes_panel
- Shows list of takes with entry count and switch/delete actions
- Includes synthesis UI with take selection and prompt
- Uses existing synthesis_formatter for diff generation

tests/test_takes_panel.py

@@ -0,0 +1,16 @@
+import pytest
+import inspect
+
+
+def test_takes_tab_replaces_placeholder():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._gui_func)
+    assert "_render_takes_placeholder" not in source, "Placeholder should be replaced"
+
+
+def test_takes_panel_has_synthesis():
+    import src.gui_2 as gui_2
+
+    source = inspect.getsource(gui_2.App._render_takes_panel)
+    assert "synthesis" in source.lower(), "Should have synthesis functionality"