fix(metrics): Add useLocalStorageSync hook for reactive changes to localstorage

[vbudhram]

Because

• The Nimbus context was not reactively updating when users changed their metrics preferences in localStorage
• Metrics opt-in/opt-out changes required a full page reload to take effect, leading to stale experiment state
• Direct localStorage reads in useEffect don't trigger re-renders when values change in the same tab

This pull request

• Adds useLocalStorageSync hook using React's useSyncExternalStore for reactive localStorage monitoring
• Dispatches custom localStorageChange events when Storage class modifies localStorage values
• Refactors NimbusContext to use useLocalStorageSync for tracking currentAccountUid and accounts data

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12738

Checklist

• My commit is GPG signed.
• [ x] If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

This change improves the reactivity of the Nimbus experimentation system by ensuring that metrics preference changes are immediately reflected without requiring page reloads. The implementation uses React 18's useSyncExternalStore API for proper external state synchronization.

[LZoog]

Code LGTM, + tested locally and it works!

[LZoog]

Nice!

[LZoog]

If I'm not signed into an account, this value is undefined and so the Nimbus experiments don't get fetched. Should this be true instead?

Also, do you plan to address the 1. and 2. points of the ticket in this PR? I think this only covers the 3rd case, but the other 2 should be easy at least if you want to do them here or in a quick follow up.

[vbudhram]

I believe 2. was already done

If I'm not signed into an account, this value is undefined and so the Nimbus experiments don't get fetched. Should this be true instead?

I can change it to true it is confusing, the experiments should get called since the check is undefined === false below

[LZoog]

Hm, when I navigate from email-first to sign-in after a fresh /clear, I get this error in the console:

I wouldn't expect this to rerender at this point, given local storage hasn't changed yet? Or maybe it did change due to Glean entry stuff and we only need to rerender on currentAccountUid and not accounts if that changes there? Looking at getAccountInfo in Signin Container, I guess we do update it with setCurrentAccount at this point, but we don't have the UID so we shouldn't rerender there yet 🤔

I don't see the error again at least when I enter the account password and the useEffect correctly runs, it seems to work, which is neat! 🎉

... Though, when I signed into an unverified account and confirmed it, when I was redirected to Settings with that account, the useEffect reran again for some reason, and accountsMetricsDisabled is set to undefined.

[vbudhram]

@LZoog I havent been able to reproduce this :/