Fix Dashboard top categories query translation error

EF Core cannot translate GroupBy with a navigation property
(Category.Name) joined via nullable FK into SQL. Split into two
queries: group by CategoryId server-side, then look up category
names in a separate query.

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

src/Purrse.Api/Services/DashboardService.cs

@@ -51,18 +51,24 @@ public class DashboardService : IDashboardService
 
         var changePercent = lastMonthSpending == 0 ? 0 : ((thisMonthSpending - lastMonthSpending) / lastMonthSpending) * 100;
 
-        var topCategories = await _db.Transactions
-            .Include(t => t.Category)
+        var spendingByCategory = await _db.Transactions
             .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, -g.Sum(t => t.Amount), 0, null))
-            .OrderByDescending(c => c.Amount)
+            .GroupBy(t => t.CategoryId)
+            .Select(g => new { CategoryId = g.Key, Total = -g.Sum(t => t.Amount) })
+            .OrderByDescending(x => x.Total)
             .Take(5)
             .ToListAsync();
 
-        var totalSpending = topCategories.Sum(c => c.Amount);
-        topCategories = topCategories.Select(c => new CategorySpending(c.CategoryName, c.Amount,
-            totalSpending > 0 ? (c.Amount / totalSpending) * 100 : 0, null)).ToList();
+        var categoryNames = await _db.Categories
+            .Where(c => spendingByCategory.Select(x => x.CategoryId).Contains(c.Id))
+            .ToDictionaryAsync(c => c.Id, c => c.Name);
+
+        var totalSpending = spendingByCategory.Sum(x => x.Total);
+        var topCategories = spendingByCategory.Select(x => new CategorySpending(
+            categoryNames.GetValueOrDefault(x.CategoryId!.Value, "Unknown"),
+            x.Total,
+            totalSpending > 0 ? (x.Total / totalSpending) * 100 : 0,
+            null)).ToList();
 
         var spendingSummary = new SpendingSummary(thisMonthSpending, lastMonthSpending, changePercent, topCategories);