fix(gui): make Quit button actually terminate the server

Signalling the process with SIGTERM/SIGINT didn't reliably shut Streamlit
down — its tornado/asyncio loop swallowed or deferred the signal, so the
browser saw the websocket drop ("Connection error") while the python
process kept running. Replace the signal with a daemon-thread
``os._exit(0)`` after a short delay so the current rerun can paint the
"shutting down" message before the process is hard-killed.

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

src/gui/components/_legacy.py

@@ -4,7 +4,8 @@ from __future__ import annotations
 
 import io
 import os
-import signal
+import threading
+import time
 from typing import Optional
 
 import pandas as pd
@@ -90,21 +91,27 @@ def hide_streamlit_chrome() -> None:
 def quit_button(label: str = "Quit app", *, key: str = "quit_app_button") -> None:
     """Render a Quit button that terminates the Streamlit server.
 
-    Streamlit has no first-class shutdown hook, so closing the browser tab
-    leaves the server (and the python process running it) alive — the user
-    has to Ctrl+C in the shell. This helper signals the Streamlit process
-    so the shell returns cleanly. SIGTERM lets Streamlit run its own
-    shutdown handlers; if that's unavailable on the platform, fall back to
-    SIGINT (the same signal Ctrl+C delivers).
+    Streamlit has no first-class shutdown hook, and signalling the
+    process (SIGTERM/SIGINT) does not reliably terminate it — Streamlit
+    installs its own handlers and the tornado/asyncio loop swallows or
+    defers the signal, so the browser sees the websocket drop while the
+    python process stays alive. To shut down cleanly we schedule
+    ``os._exit(0)`` on a daemon thread after a short delay; the delay
+    lets the current rerun finish painting the "shutting down" message
+    before the process is hard-killed.
     """
     if st.session_state.get("_app_shutting_down"):
-        st.success("App shut down. You can close this browser tab.")
+        st.success("Shutting down… you can close this browser tab.")
         st.stop()
 
     if st.button(label, key=key, type="secondary"):
         st.session_state["_app_shutting_down"] = True
-        sig = getattr(signal, "SIGTERM", signal.SIGINT)
-        os.kill(os.getpid(), sig)
+
+        def _hard_exit() -> None:
+            time.sleep(0.5)
+            os._exit(0)
+
+        threading.Thread(target=_hard_exit, daemon=True).start()
         st.rerun()