feat(android): say that work is happening, on every screen

The home screen's buttons already dimmed during an operation, but from the
list nothing moved: deleting or duplicating takes a second or two - the
container's round trip - and the screen stayed identical, as if the tap had
not registered. A veil with a turning arc now covers any screen, and it also
prevents the second tap, which would not wait any better.

Its safety net is the existing 25 s deadline: even if an operation never
hands back, the veil lifts.

The plan records where deleted entries actually are: every publish is a Gitea
commit, so the snapshot from before the deletion is still in the repository's
history, readable in its web interface and recoverable by import. Not a
screen, but a real net - and one that catches what a trash would not: an
unlucky edit, a bad import.

android-client/web/app.html

@@ -283,6 +283,19 @@
     pointer-events: none;
   }
   .toast.on { opacity: 1; }
+  /* DIRE QUE CA TRAVAILLE, sur n'importe quel ecran. Les boutons de l'accueil
+     palissaient deja pendant une operation, mais depuis la liste rien ne
+     bougeait : supprimer ou dupliquer prend une a deux secondes - le trajet du
+     conteneur - et l'ecran restait identique, comme si l'appui n'avait pas
+     pris. Le voile empeche aussi le second appui, qui n'attendrait pas mieux. */
+  .busy { position: fixed; inset: 0; z-index: 90; display: none;
+          align-items: center; justify-content: center;
+          background: rgba(0, 0, 0, .38); }
+  body.busy .busy { display: flex; }
+  .busy > div { background: var(--bg-top); border-radius: var(--radius);
+                padding: 20px 26px; display: flex; align-items: center;
+                gap: 12px; font-size: 15px; }
+  .busy svg { width: 22px; height: 22px; animation: spin .9s linear infinite; }
 </style>
 
 <div class="screen" id="unlock">
@@ -431,6 +444,7 @@
   <div class="menu" id="menu" hidden></div>
 </div>
 
+<div class="busy" id="busy"><div></div></div>
 <div class="toast" id="toast"></div>
 
 <script>
@@ -1368,6 +1382,14 @@ var PM = {
     clearTimeout(t._h);
     t._h = setTimeout(function () { t.classList.remove('on'); }, 2600);
   },
+  /// Delphi l'appelle des qu'il part travailler et des qu'il revient. Le
+  /// filet est son minuteur de 25 s : meme si une operation ne rend jamais la
+  /// main, le voile se leve.
+  setBusy: function (on) {
+    if (on) document.getElementById('busy').firstChild.innerHTML =
+      ico('loader', 'Working') + '<span>Please wait...</span>';
+    document.body.classList.toggle('busy', !!on);
+  },
   back: function () { if (view === 'detail') goList(); },
   /// La touche retour annule la selection avant de verrouiller le coffre.
   /// Sans cela, un geste d'annulation fermait le coffre - le contraire de ce