fix(close): graceful about:blank fallback + display-mode aware hint

Reported: user asked whether we can send Alt+F4 / Ctrl+W to the
browser from JavaScript to force-close a tab.

Honest answer that's now baked into the hint message: NO. Synthesized
keyboard events from page JS only reach DOM event listeners, not the
browser chrome or the OS. There is no flag, API, or trick that lets
a page close a tab the user opened themselves. The page CAN close a
window it opened (window.opener trail) or one whose display-mode is
``standalone`` (Chrome/Edge ``--app=URL``) — that's what
``python -m src.gui`` arranges, and that's the path that actually
closes the window without a manual Ctrl+W.

Improvements landed:

1. ``isStandalone(win)`` detects Chrome --app windows up front
   (``matchMedia('(display-mode: standalone)').matches``). In a
   regular tab the manual hint surfaces immediately on the
   "Close this window" click; in --app mode we only show it if the
   close attempt actually fails.

2. ``fallbackToBlank(win)`` navigates the tab to ``about:blank``
   via ``location.replace`` (no history pollution) so the user
   sees a clean empty tab instead of the farewell overlay frozen
   over Streamlit's connection-error banner. They still have to
   Ctrl+W the blank tab, but the screen is no longer a misleading
   "did it close or not?" mess. Fires 250 ms after a failed close
   in --app mode (very rare path), or 1.5 s in a regular tab so
   the user has time to read the hint.

3. Hint message rewritten in en + es to explain WHY the close is
   blocked (browser security — not something we can override), to
   acknowledge the Alt+F4 / Ctrl+W question directly (those don't
   work either, for the same reason), and to point at
   ``python -m src.gui`` as the path that gives a clean auto-close.

2220 tests pass.

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

src/i18n/packs/es.json

@@ -53,7 +53,7 @@
     "farewell_title": "DataTools se ha cerrado",
     "farewell_subtitle": "Ya puedes cerrar esta ventana.",
     "close_window_button": "Cerrar esta ventana",
-    "close_hint": "Tu navegador está bloqueando el cierre — es una restricción de seguridad para pestañas que abriste tú mismo, no algo que podamos anular. Presiona Ctrl+W (o ⌘W en Mac) para cerrar esta pestaña. Ejecuta DataTools con python -m src.gui para tener una ventana de aplicación que se cierra sola."
+    "close_hint": "Los navegadores no permiten que JavaScript cierre una pestaña que tú abriste — tampoco pueden enviar Ctrl+W ni Alt+F4 (esas combinaciones las intercepta el SO, no la página). Ejecuta DataTools con `python -m src.gui` para obtener una ventana Chrome/Edge --app que sí se cierra. Mientras tanto, esta pestaña pasará a una página en blanco en un momento — ciérrala con Ctrl+W (o ⌘W en Mac)."
   },
   "close_page": {
     "page_title": "DataTools — Cerrar",