phsl / new-api

- useHeaderBar.js
  - Wrap handlers with useCallback (logout, language/theme toggle, mobile menu)
  - Add null checks and try/catch around iframe postMessage (theme & language)
  - Keep minimal effect deps; remove unused StatusContext dispatch
  - Logo preload effect safe and scoped to `logo`

- index.jsx
  - No functional changes; locale memoization remains stable

Chore:
- Lint clean; no runtime warnings
- Verified no render loops or performance regressions