feat(nav): admin section label + role chip beside the user name

- A clearer 'Admin' section header (thicker centred rule when collapsed) above the
  admin-only modules (Scheduler/Configuration/Users), replacing the faint hairline.
- A small role chip (admin/user/demo) next to the account name — an avatar-corner dot
  when the rail is collapsed.

frontend/src/i18n/locales/en/nav.json

@@ -1,5 +1,11 @@
 {
   "primary": "Primary navigation",
   "collapse": "Collapse sidebar",
-  "expand": "Expand sidebar"
+  "expand": "Expand sidebar",
+  "adminSection": "Admin",
+  "role": {
+    "admin": "Admin",
+    "user": "User",
+    "demo": "Demo"
+  }
 }