fix(push): recover Android FCM registration

[jarynclouatre]

Summary

• override the FlutterFire Android BoM to 34.18.0, resolving Firebase Messaging 25.1.2 with the FID_ALREADY_USED registration fix
• allow FCM initialization to retry after a transient registration failure while coalescing concurrent attempts
• add a focused regression test for recovery after an initial failure

Verification

• Gradle releaseRuntimeClasspath resolves firebase-messaging:25.1.2
• Android debug APK builds successfully
• dart analyze apps/mobile (no errors or warnings; existing info diagnostics only)
• flutter test (1,529 passed, 4 skipped)