feat(banners): rotate active banners one at a time with a pager

Stacking every active banner vertically ate header height when several
were active at once. Only the highest-priority banner renders now; a
‹ 1/N › pager cycles through the rest, and all active banners are still
recorded in the notification-center history so cycled-away ones stay
reachable. Newly registered banners preempt the displayed one only when
they outrank it.

Also fix the startup flow: the restart-required banner (now priority 80)
outranks the model-folders setup warning (60), and the setup banner is
retired once a non-empty folder path is saved.

New banners.pager.* keys translated in all 9 locales.

locales/en.json

@@ -2741,6 +2741,11 @@
       "content": "Scan and manage VAE, upscaler, text encoder, CLIP vision and ControlNet files — and download them from CivitAI — from one dedicated page.",
       "enable": "Enable Other Models",
       "openSettings": "Open Settings"
+    },
+    "pager": {
+      "previous": "Previous message",
+      "next": "Next message",
+      "position": "Message {current} of {total}"
     }
   }
 }