Replace the hand-rolled model.Copy(target) pattern in every SaveXxx
with FindIndex -> in-place replace (or Add on new). Move the two
hidden defaults out of the deleted Copy bodies: Currency = "CHF"
default now lives in AccountRepository.SaveAccount; BookingDate =
ValueDate default now lives in TransactionRepository.SaveTransaction.
Delete Copy from Account, BookingRule, BookingText, SubClass, and
SplitPredefinition. Keep Transaction.Clone() (renamed from Copy, and
now includes RelatedParty) for the defensive copy in GetTransaction
and ViewService.WithDisplaySign.
Fixes a pre-existing bug where Transaction.Copy silently dropped
RelatedParty on every update save.
42 tests total, all passing.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
