Harden grade reminder notification PendingIntent by using explicit activity intent

[Copilot]

This PR addresses the code scanning finding for implicit PendingIntent usage in the Android reminder notification flow. The notification tap action now uses an explicit app activity target to remove intent hijacking risk.

• Security fix (alert Refactor Turnstile integration to popup verify with robust init and error reporting #12)

  • Updated GradeReminderNotifier.openAppIntent() to always construct an explicit Intent(appContext, MainActivity::class.java) before creating the PendingIntent.
  • Preserved existing launch behavior via FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TOP.
  • Kept immutable pending intent semantics unchanged.

• Impact scope

  • Single-file, localized change in:
    • android/app/src/main/java/com/clhs/score/reminders/GradeReminderNotifier.kt
  • No flow or UX redesign; only intent construction was tightened.

val intent = Intent(appContext, MainActivity::class.java).apply {
    flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP
}
return PendingIntent.getActivity(
    appContext,
    nextRequestCode(),
    intent,
    PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE,
)

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