add default queue for cron jobs, fix mobile sidebar scroll

Cron-scheduled workers (retention, dead link checker, pruners) were
enqueued to the default queue which wasn't configured — they never ran.
Also add overflow-y scroll and sticky positioning to admin sidebar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/admin/components.css

@@ -63,6 +63,9 @@
   z-index: 1;
   width: 16rem;
   min-height: 100%;
+  max-height: 100vh;
+  max-height: 100dvh;
+  overflow-y: auto;
   display: flex;
   flex-direction: column;
   background-color: var(--t-surface-sunken);
@@ -78,6 +81,11 @@
     position: relative;
   }
 
+  .admin-sidebar {
+    position: sticky;
+    top: 0;
+  }
+
   .admin-sidebar-overlay {
     display: none;
   }