Migrate NFS pages to HeaderPage

Always render the plugin header for page blueprints and move page-level actions into the Backstage UI header pattern for affected NFS pages.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/notifications/package.json

@@ -58,6 +58,7 @@
     "@backstage/plugin-notifications-common": "workspace:^",
     "@backstage/plugin-signals-react": "workspace:^",
     "@backstage/theme": "workspace:^",
+    "@backstage/ui": "workspace:^",
     "@material-ui/core": "^4.9.13",
     "@material-ui/icons": "^4.9.1",
     "lodash": "^4.17.21",