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/en.json

@@ -53,7 +53,7 @@
     "farewell_title": "DataTools has shut down",
     "farewell_subtitle": "You can close this window.",
     "close_window_button": "Close this window",
-    "close_hint": "Your browser is blocking the close — this is a security restriction on tabs you opened yourself, not something we can override. Press Ctrl+W (or ⌘W on Mac) to close this tab. Launch DataTools with python -m src.gui to get an auto-closing app window."
+    "close_hint": "Browsers don't let JavaScript close a tab you opened yourself — and they don't let it send Ctrl+W or Alt+F4 either (those keystrokes are intercepted by the OS, not the page). Launch DataTools with `python -m src.gui` to get a Chrome/Edge --app window that DOES close cleanly. In the meantime, this tab will fall back to a blank page in a moment — close it manually with Ctrl+W (or ⌘W on Mac)."
   },
   "close_page": {
     "page_title": "DataTools — Close",