feat: add Zod validation schemas

[dzekojohn4]

Task

Create src/utils/schemas.ts with Zod validation schemas.

Schemas to define

export const sendMoneySchema = z.object({
  recipient: z.string().length(56, 'Invalid Stellar address'),
  amount: z.string().regex(/^\d+(\.\d{1,7})?$/, 'Invalid amount'),
  asset: z.enum(['XLM', 'USDC']),
});

Acceptance Criteria

• Schema exported and usable with react-hook-form + zodResolver
• Validation messages are user-friendly