Import Preview: Available from is the zero value

[morremeyer]

Describe the bug
For import previews, the "Available From" value is always the zero value.

Expected behavior
It should be the first of the month the transaction is in.

[morremeyer]

After fixing this, update https://github.com/envelope-zero/frontend/blob/9e5ff5e2394192cc16cbf77feec869286e673ecf/cypress/e2e/transaction-import.cy.ts#L58.