The duplicate functions at lines 455-486 were redefining cacheElements and
bindEvents with wrong element IDs (kebab-case vs camelCase in HTML).
This caused 'Cannot read properties of null' error on slides app init.
- Add JavaScript to load user profile from /api/auth/me endpoint
- Save access_token to localStorage/sessionStorage on login
- Update user menu to show actual user name and email
- Toggle Sign in/Sign out based on authentication state
- Add IDs to user menu elements for dynamic updates
