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/es.json

@@ -171,6 +171,17 @@
 	"admin_users_search_placeholder": "Buscar usuarios\u2026",
 	"admin_users_empty": "No hay usuarios.",
 	"admin_users_select_prompt": "Selecciona un usuario de la lista.",
+	"admin_btn_new_group": "Nuevo grupo",
+	"admin_groups_list_title": "Todos los grupos",
+	"admin_groups_empty": "No hay grupos.",
+	"admin_groups_select_prompt": "Selecciona un grupo de la lista.",
+	"admin_groups_permission_count": "{count} permisos",
+	"admin_group_new_heading": "Crear nuevo grupo",
+	"admin_group_edit_heading": "Grupo: {name}",
+	"admin_group_updated": "Grupo guardado.",
+	"admin_group_created": "Grupo creado.",
+	"admin_groups_section_standard": "Est\u00e1ndar",
+	"admin_groups_section_administrative": "Administrativo",
 	"admin_user_new_heading": "Crear nuevo usuario",
 	"admin_user_edit_heading": "Editar usuario: {username}",
 	"admin_user_created": "Usuario creado.",