Fix precision loss: use atomic units for monetary amounts instead of Number()

[coderabbitai]

Problem

Converting monetary amounts to JavaScript Number can cause silent precision loss, which is unsafe for financial operations.

Current Implementation

In src/app/(dashboard)/s/[id]/_components/subscription-plan-preview.tsx (line ~97):

const recurringPaymentBody = {
  payee: subscriptionPlan.recipient,
  amount: Number(subscriptionPlan.amount),  // ⚠️ Unsafe conversion
  invoiceCurrency: subscriptionPlan.paymentCurrency as PayoutCurrency,
  // ...
};

Suggested Solution

Send atomic units (string or bigint) end-to-end to avoid precision loss:

const recurringPaymentBody = {
  payee: subscriptionPlan.recipient,
  amountAtomic: amount.toString(),  // Send as string
  invoiceCurrency: subscriptionPlan.paymentCurrency as PayoutCurrency,
  // ...
};

Required Changes

1. Update the recurringPaymentBody to use amountAtomic: string (or bigint)
2. Update useCreateRecurringPayment hook to accept atomic units
3. Update API types and backend handlers to accept and validate atomic unit strings
4. Compute human-readable display values from atomic units when needed

Context

• Originated in: PR feat: rework the layout to be more in line with dashboard designs #164
• Comment thread: feat: rework the layout to be more in line with dashboard designs #164 (comment)
• Requester: @bassgeta
• Acknowledgment: Carried-over technical debt

Impact

Priority: High - affects financial calculations and could lead to incorrect payment amounts.