fix(gui): keep sidebar reopenable + add clean Quit button

The chrome-hiding CSS was removing the Streamlit header wholesale,
which also took the sidebar's expand chevron with it — a collapsed
sidebar became unreopenable. Make the header transparent instead and
explicitly preserve the sidebar collapsed-control.

Also add a Quit button in the app footer that signals the Streamlit
server (SIGTERM, falling back to SIGINT) so closing the GUI returns
the shell prompt cleanly instead of leaving Python hung.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/components/__init__.py

@@ -41,6 +41,7 @@ from . import _legacy as _legacy  # noqa: F401  (keep for direct access)
 __all__ = [
     # Shared chrome / pickup / gate
     "hide_streamlit_chrome",
+    "quit_button",
     "pickup_or_upload",
     "require_normalization_gate",
     # Dedup widgets