feat(admin): add BLOG_WRITE to group permission checkbox UI

Both /admin/groups/new and /admin/groups/[id] now expose BLOG_WRITE in the
standard-permissions card so admins can grant Geschichten authoring through
the UI instead of running raw SQL. Adds Paraglide labels in de/en/es.

Closes Markus's review B1 on PR #382.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/es.json

@@ -228,6 +228,7 @@
 	"admin_perm_read_all": "Solo lectura",
 	"admin_perm_annotate_all": "Leer y anotar",
 	"admin_perm_write_all": "Leer y escribir",
+	"admin_perm_blog_write": "Escribir historias",
 	"admin_perm_admin": "Acceso completo (Admin)",
 	"admin_perm_admin_user": "Gestionar usuarios",
 	"admin_perm_admin_tag": "Gestionar etiquetas",