feat: the + joins a lone file pane's top bar, beside Обновить and above the delete button, growing in smoothly

internal/server/web/static/app.css

@@ -340,6 +340,10 @@ body.multi .area { margin-top:0; border-top-left-radius:0; }
 body.split1 .float-add { top:42px; right:16px; }
 .float-add:hover, .float-add:focus-visible { opacity:1; background:var(--panel); }
 body.multi .float-add { opacity:0; pointer-events:none; }
+.float-add { transition:opacity .25s ease, background .25s ease, top .3s ease, right .3s ease, width .3s ease, height .3s ease, border-radius .3s ease; }
+/* In a file pane's top bar: a button like its neighbours. */
+.float-add.in-files { width:34px; height:34px; font-size:19px; opacity:1; border-radius:8px; }
+.float-add.in-files:hover { background:#151a21; }
 .tabview { position:absolute; inset:8px; opacity:0; visibility:hidden; transform:translateY(4px) scale(.995);
            transition:opacity .22s ease, transform .22s ease, visibility 0s .22s; }
 /* A tab fading out takes no pointer: a drop goes to the tab coming in. */