feat: dialogs open and close smoothly, the backdrop fading with them

internal/server/web/templates/files.html

@@ -84,7 +84,6 @@
   }
   dialog { background:var(--panel); color:var(--text); border:1px solid var(--border);
            border-radius:12px; padding:0; max-width:900px; width:92vw; }
-  dialog::backdrop { background:rgba(0,0,0,.6); }
   /* A dialog takes the focus itself when it opens, rather than its first
      button, which would light up as if tabbed to. */
   dialog:focus { outline:none; }