fix(sidebar): render audit module nav group as details/summary section

Module-contributed sidebar.admin_nav_item slots are now rendered as
complete <details><summary> nav groups appended to the admin panel,
matching the same pattern as core groups (Organization, Roles, etc.).

Fixes undefined $moduleNavItemSlots by reading layoutNav early.
Merges mail log into the module-contributed Audit & logs group when
both exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/audit/i18n/default_de.json

@@ -67,5 +67,6 @@
   "Lifecycle runs (7d)": "Lifecycle-Läufe (7d)",
   "Lifecycle risks (7d)": "Lifecycle-Risiken (7d)",
   "Top lifecycle reason codes (7d)": "Top-Lifecycle-Fehlercodes (7d)",
-  "Recent lifecycle risks (7d)": "Aktuelle Lifecycle-Risiken (7d)"
+  "Recent lifecycle risks (7d)": "Aktuelle Lifecycle-Risiken (7d)",
+  "Audit & logs": "Audit & Protokolle"
 }