MintForm: validate recipient address before submission

[Ejirowebfi]

Description

MintForm.tsx does not validate the recipient address field before calling mint_tokens. An invalid address causes a confusing RPC error instead of a clear inline validation message.

Tasks

• Use isValidStellarAddress from utils/validation.ts on the recipient field
• Show an inline error message for invalid addresses
• Disable the submit button while the address is invalid
• Add a unit test covering valid and invalid address inputs

Acceptance Criteria

• Invalid Stellar address shows an inline error
• Submit button is disabled until the address is valid
• Valid address clears the error and enables submission

[xx7412421-cloud]

Hi maintainers, I'd like to take this issue if it's available.

Planned approach:

• reuse the existing Stellar address validator for the mint recipient field
• show a clear inline error before submission
• disable mint submission while the recipient address is invalid
• add a focused unit test covering valid and invalid recipient inputs

I will wait for assignment before opening a PR so the workflow stays fair.