Session replay: apply remote (server-side) masking config

[ioannisj]

Part of the replay parity tracker PostHog/posthog#30889.

Problem

Flutter replay masking is computed entirely from local Dart config. PostHogSessionReplayConfig.toMap() forwards only sampleRate to native (posthog_config.dart:597), and masking is applied Dart-side from local config in the screenshot pipeline, so a server-delivered masking payload has no path into Flutter's masking. Only sampleRate is remote-aware today.

Scope

• Plumb remote-config masking into the Dart screenshot masking pipeline.

Reference: posthog-js applies server-side masking in packages/browser/src/extensions/replay/external/lazy-loaded-session-recorder.ts (this._remoteConfig?.masking).

Parity across SDKs

• Android: Session replay: apply remote (server-side) masking config posthog-android#639
• iOS: Session replay: apply remote (server-side) masking config posthog-ios#725
• React Native: Session replay: apply remote (server-side) masking config posthog-js#4184
• Flutter: Session replay: apply remote (server-side) masking config #492 (this issue)