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/gui/components/_legacy.py

@@ -121,20 +121,27 @@ _FAREWELL_SCRIPT_TEMPLATE = """
     // Strategy: append a full-screen overlay directly to the parent's
     // document.body (Streamlit's component iframes carry
     // allow-same-origin, so cross-frame DOM access is permitted).
-    // The overlay uses z-index 2147483647 (max int32) so it sits on
-    // top of Streamlit's "Connection error" banner when the websocket
-    // drops a moment later. It's appended as a sibling of React's
-    // root, so React's re-renders won't touch it.
     //
-    // We don't navigate to a data: URL here because Chrome blocks
-    // top-frame navigation to data: URLs (anti-phishing, Chrome 60+).
+    // Closing the tab via JavaScript only works in windows JS opened —
+    // Chrome/Edge --app windows qualify; a regular browser tab does
+    // NOT, and there's no way to override that from page JS (no flag,
+    // no API, no keystroke injection — synthesized keydown events
+    // never reach the browser chrome or the OS). When close fails we
+    // navigate the window to ``about:blank`` so the user at least
+    // sees a clean blank tab instead of the connection-error overlay
+    // Streamlit shows when the websocket drops.
     //
-    // The overlay carries a manual "Close this window" button as a
-    // fallback for browsers that block the auto-close. ``window.close()``
-    // only succeeds when the tab was JS-opened (Chrome --app windows
-    // qualify; regular browser tabs do not) — the button click counts
-    // as a user gesture and gives one more chance. A short timeout
-    // reveals a Ctrl+W hint if the browser still refused.
+    // Display-mode detection (``standalone`` for --app windows,
+    // ``browser`` for regular tabs) lets us skip the futile close
+    // attempt on regular tabs and route straight to the about:blank
+    // fallback.
+    function isStandalone(win) {
+      try {
+        return win.matchMedia('(display-mode: standalone)').matches
+            || win.matchMedia('(display-mode: minimal-ui)').matches
+            || win.matchMedia('(display-mode: fullscreen)').matches;
+      } catch (e) { return false; }
+    }
     function buildOverlay(doc) {
       var overlay = doc.createElement('div');
       overlay.id = 'datatools-farewell-overlay';
@@ -160,35 +167,43 @@ _FAREWELL_SCRIPT_TEMPLATE = """
       return overlay;
     }
     function tryClose(win) {
-      // Three escalating attempts. Modern browsers only honour close()
-      // for windows that JS opened (Chrome --app windows qualify; a
-      // regular browser tab does not) — there is no JS override for
-      // that policy. The window.open('', '_self') trick used to rewrite
-      // a window's "opener" lineage and let close() through in older
-      // Chrome; modern Chrome patched it but it costs nothing to try.
+      // Escalating attempts. None of these can override the browser's
+      // close-restriction policy on regular tabs.
       try { win.close(); } catch (e) {}
-      if (win.closed) return;
+      if (win.closed) return true;
       try {
         var w = win.open('', '_self', '');
         if (w) {
           try { w.close(); } catch (e) {}
         }
       } catch (e) {}
-      if (win.closed) return;
+      if (win.closed) return true;
       try { win.top.close(); } catch (e) {}
+      return win.closed;
+    }
+    function fallbackToBlank(win) {
+      // Navigate to about:blank so the user sees a clean empty tab
+      // instead of the farewell overlay frozen on a connection-error
+      // page. They can still close the tab themselves (Ctrl+W /
+      // ⌘W / clicking the tab's X). Done as a single fast call — no
+      // history entry pollution because location.replace doesn't
+      // push to history.
+      try { win.location.replace('about:blank'); } catch (e) {}
     }
     function wireClose(doc, win) {
       var btn = doc.getElementById('datatools-close-btn');
       if (!btn) return;
       btn.onclick = function () {
-        tryClose(win);
-        // If after 250ms the window is still here, the browser
-        // blocked the close — show the manual-close hint.
+        var standalone = isStandalone(win);
+        if (tryClose(win)) return;
+        // Close failed (or definitely will fail in a regular tab).
+        // Surface the hint immediately, then redirect to about:blank
+        // after a short delay so the user has a moment to read why.
+        var hint = doc.getElementById('datatools-close-hint');
+        if (hint) hint.style.display = 'block';
         setTimeout(function () {
-          if (win.closed) return;
-          var hint = doc.getElementById('datatools-close-hint');
-          if (hint) hint.style.display = 'block';
-        }, 250);
+          if (!win.closed) fallbackToBlank(win);
+        }, standalone ? 250 : 1500);
       };
     }
     try {
@@ -199,7 +214,9 @@ _FAREWELL_SCRIPT_TEMPLATE = """
       }
       wireClose(doc, win);
       // Auto-close attempt on first paint — succeeds in Chrome --app
-      // windows, no-ops on regular tabs.
+      // windows, fails silently on regular tabs (and we don't redirect
+      // automatically here; the manual button drives that path so the
+      // user is in control).
       tryClose(win);
     } catch (e) {
       // Cross-origin access denied (shouldn't happen given Streamlit's