[F4:TF5] Create Transaction to Increase Expense Saved Amount & Update Total Balances

[timebox: , complexity: ]

As a user, when the expense edit form is open, I want to move money between this expense’s saved value and the “Extra Money” value, by adding a transaction to that expense’s transaction list, that will in turn reduce or increase the amount in “Extra Money”.

No files or tests are named. Start by tracing the expense edit form, the expense transaction list, and the calculations for saved value, Extra Money, and total balances; done means a transaction can move money between the expense and Extra Money while all displayed balances update consistently.

Written by the indexing model from the issue text.