Add form validation for payment methods and required fields

[fifle]

This PR implements form validation for payment methods and required fields:

• Validates IBAN field when internet-bank methods (Swedbank, SEB, LHV, Coop) are enabled
• Validates required fields (campaign details, bank details name, payee's name)
• Validates SEB UID tokens when SEB method is enabled (requires at least one UID token)
• Validates credit card methods when enabled (requires corresponding field values)
• Displays inline error messages with proper styling
• Shows which step needs to be configured when validation fails
• Prevents form submission when validation fails
• Navigates to the step with errors when validation fails

This addresses the issue where required fields validation for internet-banks and credit cards was not working properly.