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>

tests/gui/test_smoke.py

@@ -61,7 +61,7 @@ EXPECTED_SUBSTRINGS: dict[str, dict[str, str]] = {
     "7_Multi_File_Merger":   {"en": "Combine Files",     "es": "Combine Files"},
     "8_Validator_Reporter":  {"en": "Quality Check",     "es": "Quality Check"},
     "9_Pipeline_Runner":     {"en": "Automated",         "es": "Automated"},
-    "99_Close":              {"en": "Close DataTools",   "es": "Cerrar DataTools"},
+    "99_Close":              {"en": "Shutting down",     "es": "Cerrando"},
 }