Add a bank feed

Gives one bank or card account its Bank Feeds card. Pass exactly one of: chartAccountId (any active asset or liability account in the Chart of Accounts; its register is reused or created, and no new chart account is made), an existing bankAccountId, or newAccount fields. mode tells the caller what comes next: bank opens bank sign-in for this account, files opens the CSV upload. A feed can be connected to a bank, used with files only, or both. Adding a feed that already shows is a no-op (alreadyExisted=true); adding a hidden feed shows it again.