feat(invites): show empty state when no groups exist in invite form

When groups load successfully but the list is empty, render a quiet
"Keine Gruppen vorhanden." message rather than a blank section that
leaves users uncertain whether groups failed to load.

Adds admin_new_invite_no_groups i18n key to de/en/es.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -768,6 +768,7 @@
 	"admin_new_invite_prefill_email": "E-Mail vorausfüllen (optional)",
 	"admin_new_invite_expires": "Ablaufdatum (optional)",
 	"admin_new_invite_groups": "Gruppen (optional)",
+	"admin_new_invite_no_groups": "Keine Gruppen vorhanden.",
 	"admin_invite_groups_load_error": "Gruppen konnten nicht geladen werden. Die Einladung kann ohne Gruppenauswahl erstellt werden.",
 	"admin_invite_created_title": "Einladung erstellt",
 	"admin_invite_created_desc": "Teile diesen Link mit der einzuladenden Person:",

frontend/messages/en.json

@@ -768,6 +768,7 @@
 	"admin_new_invite_prefill_email": "Pre-fill email (optional)",
 	"admin_new_invite_expires": "Expiry date (optional)",
 	"admin_new_invite_groups": "Groups (optional)",
+	"admin_new_invite_no_groups": "No groups exist.",
 	"admin_invite_groups_load_error": "Groups could not be loaded. The invite can still be created without group assignment.",
 	"admin_invite_created_title": "Invite created",
 	"admin_invite_created_desc": "Share this link with the person you are inviting:",

frontend/messages/es.json

@@ -768,6 +768,7 @@
 	"admin_new_invite_prefill_email": "Prellenar correo (opcional)",
 	"admin_new_invite_expires": "Fecha de vencimiento (opcional)",
 	"admin_new_invite_groups": "Grupos (opcional)",
+	"admin_new_invite_no_groups": "No hay grupos disponibles.",
 	"admin_invite_groups_load_error": "No se pudieron cargar los grupos. La invitación puede crearse sin asignar grupos.",
 	"admin_invite_created_title": "Invitación creada",
 	"admin_invite_created_desc": "Comparte este enlace con la persona invitada:",