Let default splitting options be stored in database

[Uli-Z]

See Issue #327 I corrected this behavior so that default splitting options are stored group-wide in the database.

Summary

• Introduced a new defaultSplittingOptions JSON column in the Group table via migration so groups can store their default split mode server‑side
• Updated the Prisma model to include this optional field, enabling persistence of these defaults across devices
• Exposed an updateDefaultSplittingOptions mutation on the groups router to modify these settings through TRPC
• Reworked the ExpenseForm to retrieve defaults from the group data and send updates via the new mutation instead of localStorage