ui(gui): say 'window' instead of 'browser tab' in shutdown copy

Update the Close page intro, the shutdown overlay, and the toast so
they all read "you can close this window" — clearer for users running
the app in a dedicated browser window rather than a tab.

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

src/gui/components/_legacy.py

@@ -105,7 +105,7 @@ _FAREWELL_SCRIPT = """
         'border-radius:12px;background:#161922;max-width:480px;">' +
         '<h1 style="margin:0 0 8px 0;font-weight:600;letter-spacing:-0.01em;">' +
         'DataTools has shut down</h1>' +
-        '<p style="opacity:0.7;margin:0;">You can close this browser tab.</p>' +
+        '<p style="opacity:0.7;margin:0;">You can close this window.</p>' +
         '</div>';
       return overlay;
     }
@@ -143,7 +143,7 @@ def quit_button(label: str = "Quit app", *, key: str = "quit_app_button") -> Non
     if st.session_state.get("_app_shutting_down"):
         from streamlit.components.v1 import html as _components_html
         _components_html(_FAREWELL_SCRIPT, height=0)
-        st.success("Shutting down… you can close this browser tab.")
+        st.success("Shutting down… you can close this window.")
         st.stop()
 
     if st.button(label, key=key, type="secondary"):