refactor: avoid synchronous state updates in useEffect

[JasminaPiric]

What this PR does / why we need it:

Fixes a cascading render warning caused by calling handleLogin synchronously inside a mounting effect.

Changes:

• Decoupled the API login logic from the manual UI loading trigger.
• Initialized isLoggingIn state based on URL parameters to prevent an immediate rerender on mount.
• Extracted performLogin to handle the side effect without triggering synchronous state updates.

Additional documentation e.g. usage docs, diagrams, reviewer notes, etc.:

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