Complete Job modal: ask powder usage once per color, not per item/coat

The modal was showing one row per coat per item, so a job with 5 items
each with 2 coats of the same powder produced 10 identical input rows.

Now groups by unique InventoryItemId and shows one row per powder color
for the whole job. The controller distributes the entered total across
coats proportionally by their estimated PowderToOrder so per-coat
reporting data is preserved. A single inventory transaction is created
per powder (net of any pre-logged scan credit).

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

src/PowderCoating.Application/DTOs/Job/JobDtos.cs

@@ -389,7 +389,7 @@ public class CompleteJobDto
 {
     public int JobId { get; set; }
     public decimal? ActualTimeSpentHours { get; set; }
-    public List<JobItemCoatUsageDto> CoatUsages { get; set; } = new();
+    public List<JobPowderUsageDto> PowderUsages { get; set; } = new();
     public bool SendEmailToCustomer { get; set; } = false;
 }
 
@@ -400,10 +400,10 @@ public class SendJobSmsRequest
     public string Message { get; set; } = string.Empty;
 }
 
-// DTO for tracking actual powder usage per coat
-public class JobItemCoatUsageDto
+// DTO for tracking actual powder usage per inventory item (color) for the whole job
+public class JobPowderUsageDto
 {
-    public int JobItemCoatId { get; set; }
+    public int InventoryItemId { get; set; }
     public decimal? ActualPowderUsedLbs { get; set; }
 }