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>

config/config.exs

@@ -107,7 +107,7 @@ config :berrypod, Oban,
        {"0 1 * * *", Berrypod.ActivityLog.PruneWorker}
      ]}
   ],
-  queues: [images: 2, sync: 1, checkout: 1, newsletter: 1]
+  queues: [default: 3, images: 2, sync: 1, checkout: 1, newsletter: 1]
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.