feat: a spinner on the save button while a file saves

internal/server/web/templates/files.html

@@ -63,6 +63,11 @@
   dialog::backdrop { background:rgba(0,0,0,.6); }
   .dlg-h { padding:14px 18px; border-bottom:1px solid var(--border); display:flex; align-items:center; gap:10px; }
   .dlg-b { padding:16px 18px; }
+  #save { display:inline-flex; align-items:center; gap:8px; }
+  #save.saving { opacity:.85; cursor:progress; }
+  #save.saving::before { content:''; width:12px; height:12px; border:2px solid rgba(255,255,255,.35); border-top-color:#fff;
+                         border-radius:50%; animation:spin .7s linear infinite; }
+  #save[hidden] { display:none; }
   /* File viewer: the code editor, a picture / video / sound, or the hex editor. */
   #viewer { max-width:1100px; }
   #viewer .dlg-h { flex-wrap:wrap; }
@@ -360,12 +365,18 @@
   const fsURL = (what, path) => '/api/panel/agents/' + encodeURIComponent(agentID) + '/fs/' + what + '?path=' + encodeURIComponent(path);
   // Writes a file as it is, with no base64: the request body is the file.
   async function putFile(path, body) {
+    // The save button turns into a small spinner meanwhile, and takes no
+    // second click.
+    const btn = $('save');
+    if (btn.classList.contains('saving')) return {ok: false, error: 'сохранение уже идёт'};
+    btn.classList.add('saving');
+    btn.disabled = true;
     setBusy(true);
     try {
       const r = await fetch(fsURL('upload', path), {method: 'POST', headers: {'Content-Type': 'application/octet-stream'}, body});
       return (r.headers.get('Content-Type') || '').startsWith('application/json') ? await r.json() : {ok: false, error: 'HTTP ' + r.status};
     } catch (e) { return {ok: false, error: 'ошибка сети'}; }
-    finally { setBusy(false); }
+    finally { setBusy(false); btn.classList.remove('saving'); btn.disabled = false; }
   }
 
   const IMG = /\.(png|jpe?g|gif|webp|bmp|ico|avif|svg)$/i, VID = /\.(mp4|m4v|webm|ogv|mov)$/i, AUD = /\.(mp3|wav|ogg|oga|m4a|aac|flac|opus)$/i;