Fix Dashboard query: replace Math.Abs with negation in GroupBy

Math.Abs() inside a GroupBy().Select() cannot be translated to
SQL by EF Core. Since the query already filters for Amount < 0,
negating the sum produces the same result and translates cleanly.

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

src/Purrse.Api/Services/DashboardService.cs

@@ -55,7 +55,7 @@ public class DashboardService : IDashboardService
             .Include(t => t.Category)
             .Where(t => t.Account.UserId == userId && t.Date >= startOfMonth && t.Amount < 0 && !t.IsVoid && t.CategoryId.HasValue)
             .GroupBy(t => new { t.CategoryId, CategoryName = t.Category!.Name })
-            .Select(g => new CategorySpending(g.Key.CategoryName, Math.Abs(g.Sum(t => t.Amount)), 0, null))
+            .Select(g => new CategorySpending(g.Key.CategoryName, -g.Sum(t => t.Amount), 0, null))
             .OrderByDescending(c => c.Amount)
             .Take(5)
             .ToListAsync();