[Optimization] Memoize OAuth config and separate theme effects in Login screen

[seer-by-sentry]

Summary of the problem

Fixes HCB-MOBILE-2E. The issue was that: Unstable useEffect dependency (isDark) causes race condition, triggering duplicate exchangeCodeAsync call with one-time-use authorization code.

Describe your changes

The fix addresses an unstable useEffect dependency (isDark) that was causing race conditions and duplicate calls to exchangeCodeAsync.

• Memoized the configuration object for useAuthRequest using useMemo to prevent unnecessary re-initialization of the OAuth flow when the color scheme changes.
• Separated the logic for setting the system status bar background color into its own useEffect hook, isolating theme-related side effects.
• Removed isDark from the dependencies of the main OAuth response handling useEffect.

This fix was generated by Seer in Sentry, triggered by Mohamad Mortada. 👁️ Run ID: 6672328.

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Checklist

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• Tested by submitter before requesting review (Does it work in development iOS/android? )