Hide admin navigation from non-admins

[BraydenLangley]

Summary

• render the Admin navigation item only when passive server status confirms the current session is an admin
• keep the item hidden while status is loading and for guest/non-admin sessions
• do not initiate wallet authentication or request an identity key during ordinary browsing
• preserve direct admin routes and server-side authorization

Validation

• npm test (10 files, 29 tests)
• npm run lint
• npm run typecheck
• npm run build
• git diff --check