Include Merchant relationship when loading transactions

Ensure merchant data is eagerly loaded when querying transactions
on the Transactions page to avoid N+1 queries.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap/Pages/Transactions.cshtml.cs

@@ -54,6 +54,7 @@ namespace MoneyMap.Pages
                     .ThenInclude(c => c!.Account)
                 .Include(t => t.Account)
                 .Include(t => t.TransferToAccount)
+                .Include(t => t.Merchant)
                 .AsQueryable();
 
             // Apply filters