feat: add optional verifier address field to CreateVault

[Joseph-1-Duro]

close #292

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[1nonlypiece]

CreateVault picked up a review-step flow on main that conflicts with this PR's inline verifier-address change. could you rebase and re-apply your field on top? then i'll take it. https://discord.gg/xvNAvMJf

[1nonlypiece]

create vault has since moved to a review-flow architecture on main, and this rewrites the same component to add verifierAddress in a way that drops that flow, so it conflicts. could you rebase on latest main and add the verifier field on top of the current create-vault? grab me in https://discord.gg/xvNAvMJf if you need a hand.

[GBOYEE]

@GBOYEE has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Implement following existing patterns and conventions. Add tests, ensure CI passes, and document any new API surface.

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