feat: Update for Flutter SDK 2.4.6

[dcrawbuck]

Update Flutter SDK documentation to version 2.4.6, including the new Android onBackPressed callback and SubscriptionStatus.isActive convenience property.

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Note

Bumps Flutter SDK docs to 2.4.6 and adds docs/examples for Android PaywallOptions.onBackPressed and the SubscriptionStatus.isActive convenience property.

• Version bump
  • Update SdkLatestVersion to 2.4.6 in content/docs/flutter/index.mdx and content/docs/flutter/sdk-reference/index.mdx.
• Paywall options (Android)
  • Add PaywallOptions.onBackPressed callback; document in signature, parameter table, and usage example in sdk-reference/PaywallOptions.mdx.
• Subscription status
  • Introduce SubscriptionStatus.isActive convenience property; add section in sdk-reference/subscriptionStatus.mdx and update quickstart and example code to use status.isActive in quickstart/tracking-subscription-state.mdx.

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