EasyInvoice - Fix Dashboard stat cards to display totals separately per currency

[coderabbitai]

Problem

The Dashboard stat cards currently sum amounts across different currencies without conversion, producing meaningless totals. For example, adding $100 USD + €50 EUR directly results in an incorrect total of "150".

This issue was identified during the review of PR #98 where the new Subscription components correctly implement multi-currency aggregation by showing separate totals for each currency.

Affected Components

Based on code analysis, the following Dashboard components have this currency aggregation issue:

• Dashboard Subscriptions (src/components/dashboard/subscriptions.tsx)
• Dashboard Invoices Received (src/components/dashboard/invoices-received.tsx)
• Dashboard Invoices Sent (src/components/dashboard/invoices-sent.tsx)

Solution

Update the Dashboard stat cards to follow the same pattern implemented in the Subscription components in PR #98:

1. Replace single-currency StatCard with MultiCurrencyStatCard for revenue/amount totals
2. Implement proper currency aggregation using BigNumber for precision:

   const totalRevenue = payments.reduce((acc, payment) => {
     const currency = payment.currency;
     const amount = BigNumber.from(payment.amount);
     
     if (acc[currency]) {
       acc[currency] = acc[currency].add(amount);
     } else {
       acc[currency] = amount;
     }
     return acc;
   }, {} as Record<string, BigNumber>);

3. Display separate totals per currency instead of one incorrect sum

Context

• Original Issue: Identified in PR feat: add active subscribers table and payments table #98 review
• PR Reference: feat: add active subscribers table and payments table #98
• Comment Reference: feat: add active subscribers table and payments table #98 (comment)
• Reported by: @MantisClone

Acceptance Criteria

• Dashboard stat cards show separate totals for each currency
• No more mixing of different currencies in single totals
• Consistent implementation across all Dashboard components
• Use MultiCurrencyStatCard component like in Subscription components