web UI: hidden dialog must beat the flex display

.dlg-back's display:flex overrode the hidden attribute, so the empty
confirm backdrop (titleless, two dead buttons) was always visible on
boot. .dlg-back[hidden] { display: none } restores the toggle.

src/web/ui.html

@@ -94,6 +94,7 @@
   .jjob .jmsg { color: var(--fg); }
   .jobs-empty { color: var(--muted); font-size: 12.5px; padding: 6px 0; }
   /* confirm dialog */
+  .dlg-back[hidden] { display: none; }
   .dlg-back {
     position: fixed; inset: 0; background: rgba(0,0,0,.5); z-index: 10;
     display: flex; align-items: center; justify-content: center; animation: fadeIn .15s ease;