Best practices in Flutter SDK

Avoid lost premium access, missing attribution, and intermittent #2002 errors by calling Adapty SDK methods in the right order.

Fetch Adapty paywalls reliably: timing, caching, and fallback patterns for Flutter.

Wait briefly for Apple Ads attribution before showing the paywall on first launch in Flutter, falling back to the default audience on timeout. Uses AdaptyProfile.appliedAttributionSources.