## Summary
- Makes `bank_id` nullable for all account types in both
`StoreAccountRequest` and `UpdateAccountRequest` (previously it was only
nullable for real estate)
- Shows the bank combobox field for all account types in the
`AccountForm` component, including real estate
- Removes the `required` attribute from the hidden bank input and the
logic that cleared bank selection when switching to real estate type
## Test changes
- Updated "validates required fields" test to no longer expect `bank_id`
as a required field
- Added "can create a new account without a bank" test in `AccountTest`
- Updated `RealEstateTest` to verify non-real-estate accounts can also
be created without a bank (was previously asserting the opposite)
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