Centralize alarm PendingIntent identities

[Rahul-2k4]

Summary

• centralize alarm PendingIntent request codes and creation in one helper
• reuse that helper from MainActivity and BootReceiver so scheduling and cancellation target the same identities
• cancel the legacy boot-restored alarm identity alongside the current alarm identity
• add a native regression test for the stable PendingIntent request-code contract

Testing

• branch diff against main reduced to the intended 4 files
• native unit test remains blocked on current main in this environment because the repo still uses the removed imperative Flutter Gradle plugin loader

Closes #887