feat(gui): one-click Close in its own bottom sidebar section

Close is now a direct shutdown trigger: visiting the Close page (the
sidebar entry) fires shutdown_app() immediately — no confirm step, no
intermediate body. The farewell overlay paints and os._exit(0) lands
~1s later from a daemon thread.

Layout: Close moved into its own bottom-of-sidebar section so the
destructive action is visually separated from Account/Activate.

- New shutdown_app() in components/_legacy.py replaces quit_button.
  os._exit thread is skipped when "pytest" is in sys.modules so the
  test suite doesn't suicide on rendering 99_Close.
- pages/99_Close.py shrinks to set_page_config + chrome + shutdown_app.
- app.py nav grows a new "Close" section header (new
  nav.section_close key in en/es packs) pinned at the bottom of the
  navigation dict.

Tests updated:
- TestQuitButtonRenders → TestClosePageShutsDownImmediately.
  Assert the shutdown caption renders + no confirm button exists.
- test_smoke EXPECTED_SUBSTRINGS["99_Close"] now pins
  "Shutting down" / "Cerrando" (the visible page body) instead of
  the removed page title.

2008 tests pass.

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

src/gui/pages/99_Close.py

@@ -1,9 +1,9 @@
-"""Close — shut the DataTools server down cleanly.
+"""Close — shut the DataTools server down immediately on visit.
 
-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.
+Sidebar nav entry. Clicking it routes here, and the page's render
+fires :func:`shutdown_app` directly — there is no confirm step. The
+browser sees the farewell overlay; the Python process terminates a
+moment later via ``os._exit``.
 """
 
 from __future__ import annotations
@@ -17,7 +17,7 @@ _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
+from src.gui.components import hide_streamlit_chrome, shutdown_app
 from src.i18n import t
 
 st.set_page_config(
@@ -25,14 +25,6 @@ st.set_page_config(
     page_icon="🛑",
     layout="wide",
 )
-
 hide_streamlit_chrome()
 
-st.title(t("close_page.title"))
-st.caption(t("close_page.caption"))
-st.divider()
-
-st.markdown(t("close_page.body"))
-
-st.write("")
-quit_button(label=t("close_page.button"), key="quit_app_button_page")
+shutdown_app()