Limit dashboard spending to top 10 categories and fix nav double-highlight

Show top 10 categories in spending breakdown with label indicating the limit.
Fix account-scoped transactions highlighting both Accounts and Transactions
nav items by excluding sub-route match when account-transactions is active.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Services/DashboardService.cs

@@ -56,6 +56,7 @@ public class DashboardService : IDashboardService
             .GroupBy(t => t.CategoryId)
             .Select(g => new { CategoryId = g.Key, Total = -g.Sum(t => t.Amount) })
             .OrderByDescending(x => x.Total)
+            .Take(10)
             .ToListAsync();
 
         var categoryIds = spendingByCategory.Where(x => x.CategoryId.HasValue).Select(x => x.CategoryId!.Value);