feat(admin/groups): add groups entity with master-detail sub-routes

Creates the full groups section under /admin/groups/:
- +layout.server.ts: loads groups list via GET /api/groups
- GroupsListPanel.svelte: left list panel (name + permission count, active state)
- +layout.svelte: composes list panel + children slot
- +page.svelte: empty selection prompt
- [id]/+page.server.ts: update (PATCH) and delete actions
- [id]/+page.svelte: edit detail panel with Standard/Administrative permission sections
- new/+page.svelte and +page.server.ts: create group form

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

frontend/messages/de.json

@@ -171,6 +171,17 @@
 	"admin_users_search_placeholder": "Benutzer suchen\u2026",
 	"admin_users_empty": "Keine Benutzer vorhanden.",
 	"admin_users_select_prompt": "W\u00e4hle einen Benutzer aus der Liste.",
+	"admin_btn_new_group": "Neue Gruppe",
+	"admin_groups_list_title": "Alle Gruppen",
+	"admin_groups_empty": "Keine Gruppen vorhanden.",
+	"admin_groups_select_prompt": "W\u00e4hle eine Gruppe aus der Liste.",
+	"admin_groups_permission_count": "{count} Berechtigungen",
+	"admin_group_new_heading": "Neue Gruppe anlegen",
+	"admin_group_edit_heading": "Gruppe: {name}",
+	"admin_group_updated": "Gruppe gespeichert.",
+	"admin_group_created": "Gruppe erstellt.",
+	"admin_groups_section_standard": "Standard",
+	"admin_groups_section_administrative": "Administrativ",
 	"admin_user_new_heading": "Neuen Benutzer anlegen",
 	"admin_user_edit_heading": "Benutzer bearbeiten: {username}",
 	"admin_user_created": "Benutzer wurde erstellt.",