fix(frontend): persist UI locale so new tabs keep the selected language

[soroush5]

Fixes #5599.

Switching language with the locale switcher only called i18n.activate() in memory — the choice was never stored anywhere. Every new tab boots through setUpApp, which activates the server default (or the browser language), so the new tab always came up in the default language instead of the one just selected.

This stores the activated locale in localStorage and reads it back at boot, keeping the existing precedence for first-time visitors: stored choice, then APP_DEFAULT_LANGUAGE, then the browser language. If storage is unavailable the app behaves exactly as before.

How to test:

1. npx jest src/utilities/__tests__/i18nPersistence.test.js in frontend — 4 tests, all fail without the fix and pass with it.
2. Manual: switch to Français, open any internal link in a new tab — the new tab stays in French. Verified the same flow in headless Chromium against the real i18n.config module: before the fix the second tab read null, after the fix it reads fr.
3. src/utilities + LocaleSwitcher suites (27 tests) still pass.

[lcampbell2]

/gcbrun