fix: await useAuthEmulator before completing Yust.initialize

[CLNMR]

Summary

• FirebaseAuth.useAuthEmulator(host, port) returns a Future<void>; on web the underlying JS connectAuthEmulator doesn't take effect synchronously, so auth calls (e.g. signInWithMicrosoft) started before that future resolves hit production endpoints instead of the emulator.
• Exposes a ready future on YustAuthService and awaits it from Yust.initialize, so callers can rely on the emulator being fully wired by the time initialization returns.
• The Dart-only and mocked variants get a no-op ready to keep the interface symmetric.

Test plan

• In a Flutter web host app that calls Yust.initialize(emulatorAddress: 'localhost', ...), federated sign-ins (Microsoft/Google) now hit http://localhost:9099/emulator/auth/handler... instead of the production OAuth handler at <authDomain>/__/auth/handler.
• In a Dart-only host (e.g. backend services), Yust.initialize still completes immediately — ready is a pre-completed Future.value() there.
• Mocked tests still pass.

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