Allow mobile notification grouping for notification spam

(#3593) Feature Open: In progress mobile notifications

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Comment by Dinosaur Gnatalie
Dinosaur Gnatalie 1 vote Merged from #3439
Can reproduce CR - Notifications were properly grouped, but each triggered a notification sound individually despite rapid-firing several messages in a row. Three separate trials returned the same results. Version 2026.10.100, Beta 2, Channel Beta, Build 2591, Android 16 (arm64), Device Google Pixel 9 Pro, Locale en-US, Push fcm, Flutter 3.47.6, Dart 3.13.5