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")
 
 
 # ---------------------------------------------------------------------------