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/pages/99_Close.py

@@ -33,9 +33,8 @@ 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."
+    "Any unsaved work in other tools will be lost. Once the app shuts "
+    "down you can close this window."
 )
 
 st.write("")