Add Custom Powder Order line item and fix CSV import FinalPrice crash

Custom powder/incoming powder material cost now flows into a separate
auto-generated 'Custom Powder Order' line item instead of rolling into
individual item prices, so users can add shipping charges before the
customer sees the total. A dashed yellow preview card in the wizard
shows the material cost and lets users edit the total (including shipping)
before saving. After first save the price is user-owned.

Also fixes a fatal CSV import crash when FinalPrice contains a non-numeric
value (e.g. 'false' from a spreadsheet formula): the job CSV importer now
streams rows one at a time with a lenient decimal converter, treating bad
values as $0 with a per-row warning instead of aborting the entire import.

Updated HelpKnowledgeBase.cs and Help articles (Jobs, Quotes) with
Custom Powder Order behavior and a new Data Import / Export section.

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

tests/PowderCoating.UnitTests/PricingCalculationServiceTests.cs

@@ -327,11 +327,13 @@ public class PricingCalculationServiceTests
 
         var result = await service.CalculateQuoteItemPriceAsync(item, companyId: 1);
 
-        Assert.Equal(10m, result.MaterialCost);
+        // Custom powder material ($10) is excluded from the item price — it moves to the
+        // auto-generated "Custom Powder Order" line item so users can add shipping.
+        Assert.Equal(0m, result.MaterialCost);
         Assert.Equal(30m, result.LaborCost);
         Assert.Equal(0m, result.EquipmentCost);
-        Assert.Equal(70m, result.UnitPrice);
-        Assert.Equal(140m, result.TotalPrice);
+        Assert.Equal(65m, result.UnitPrice);   // catalog $50 + prep $15/unit
+        Assert.Equal(130m, result.TotalPrice);
     }
 
     [Fact]