feat(gui): promote Quit to a 'Close' menu item in the sidebar nav

Move the shutdown control out of the inline sidebar widget and into
its own page (pages/99_Close.py), so it appears in the sidebar nav
alongside the tool pages. An explicit confirm button on the page
prevents accidental nav clicks from killing a live session.

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

src/gui/components/_legacy.py

@@ -72,16 +72,8 @@ footer {
 
 
 def hide_streamlit_chrome() -> None:
-    """Inject CSS to hide Streamlit's default header, menu, and footer.
-
-    Also renders a Quit button at the bottom of the sidebar so every page
-    that hides the default chrome still has a clean way to shut the
-    server down.
-    """
+    """Inject CSS to hide Streamlit's default header, menu, and footer."""
     st.markdown(_HIDE_CHROME_CSS, unsafe_allow_html=True)
-    with st.sidebar:
-        st.markdown("---")
-        quit_button(key="quit_app_button_sidebar")
 
 
 # ---------------------------------------------------------------------------

src/gui/pages/99_Close.py

@@ -0,0 +1,42 @@
+"""Close — shut the DataTools server down cleanly.
+
+Lives in the sidebar nav alongside the tool pages so users have a
+discoverable way to terminate the local Streamlit process without
+having to Ctrl+C in the shell. An explicit confirm step prevents an
+accidental sidebar click from killing a session mid-work.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import streamlit as st
+
+_project_root = Path(__file__).resolve().parent.parent.parent.parent
+if str(_project_root) not in sys.path:
+    sys.path.insert(0, str(_project_root))
+
+from src.gui.components import hide_streamlit_chrome, quit_button
+
+st.set_page_config(
+    page_title="DataTools — Close",
+    page_icon="🛑",
+    layout="wide",
+)
+
+hide_streamlit_chrome()
+
+st.title("🛑 Close DataTools")
+st.caption("Shut down the local app and free the terminal.")
+st.divider()
+
+st.markdown(
+    "Clicking the button below will terminate the DataTools server. "
+    "Any unsaved work in other tools will be lost. The browser tab "
+    "will show a connection-error message — that's expected; you can "
+    "close the tab once it appears."
+)
+
+st.write("")
+quit_button(label="Close the app", key="quit_app_button_page")