feat(tier4): Integrate patch generation into GUI workflow

- Add patch_callback parameter throughout the tool execution chain
- Add _render_patch_modal() to gui_2.py with colored diff display
- Add patch modal state variables to App.__init__
- Add request_patch_from_tier4() to trigger patch generation
- Add run_tier4_patch_callback() to ai_client.py
- Update shell_runner to accept and execute patch_callback
- Diff colors: green for additions, red for deletions, cyan for headers
- 36 tests passing

tests/test_tier4_interceptor.py

@@ -130,5 +130,5 @@ def test_gemini_provider_passes_qa_callback_to_run_script() -> None:
      base_dir=".",
      qa_callback=qa_callback
     )
-    # Verify _run_script received the qa_callback
-    mock_run_script.assert_called_with("dir", ".", qa_callback)
+    # Verify _run_script received the qa_callback and patch_callback
+    mock_run_script.assert_called_with("dir", ".", qa_callback, None)