feat: add input validation in create form matching contract guards

[lekanay2005-coder]

Summary

Add client-side input validation to the create payment form that mirrors the contract's validation guards.

Acceptance Criteria

• Non-empty recipients check
• recipients.length === shares.length check
• Positive amount check
• No zero shares check
• No duplicate recipients check
• Error messages shown inline next to the relevant field

Tech Stack

React, TypeScript, Vite