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/de.json

@@ -2741,6 +2741,11 @@
             "content": "Scannen und verwalten Sie VAE-, Upscaler-, Text-Encoder-, CLIP-Vision- und ControlNet-Dateien und laden Sie sie von CivitAI herunter, alles auf einer eigenen Seite.",
             "enable": "Weitere Modelle aktivieren",
             "openSettings": "Einstellungen öffnen"
+        },
+        "pager": {
+            "previous": "Vorherige Mitteilung",
+            "next": "Nächste Mitteilung",
+            "position": "Mitteilung {current} von {total}"
         }
     }
 }