refactor(ui): polish admin sidebar nav groups and audit labels

Use short i18n keys for audit nav labels (nav.audit.*) with translations
in both language files. Restyle admin sidebar group icons with colored
pill backgrounds per group. Adjust details summary line-height and
active-state border color to use group icon color.

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

i18n/default_de.json

@@ -977,6 +977,11 @@
   "All API tokens will be revoked immediately.": "Alle API-Tokens werden sofort widerrufen.",
   "API token default lifetime is invalid": "Standardlaufzeit für API-Tokens ist ungültig",
   "API token max lifetime is invalid": "Maximale Laufzeit für API-Tokens ist ungültig",
+  "nav.logs.mail": "E-Mail",
+  "nav.audit.api": "API",
+  "nav.audit.system": "System",
+  "nav.audit.imports": "Importe",
+  "nav.audit.user_lifecycle": "Benutzer-Lifecycle",
   "API audit": "API-Protokolle",
   "API audit logs": "API-Protokolle",
   "Request ID": "Request-ID",

i18n/default_en.json

@@ -977,6 +977,11 @@
   "All API tokens will be revoked immediately.": "All API tokens will be revoked immediately.",
   "API token default lifetime is invalid": "API token default lifetime is invalid",
   "API token max lifetime is invalid": "API token max lifetime is invalid",
+  "nav.logs.mail": "E-mail",
+  "nav.audit.api": "API",
+  "nav.audit.system": "System",
+  "nav.audit.imports": "Imports",
+  "nav.audit.user_lifecycle": "User lifecycle",
   "API audit": "API logs",
   "API audit logs": "API logs",
   "Request ID": "Request ID",