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>

src/PowderCoating.Application/DTOs/Quote/QuoteDtos.cs

@@ -884,4 +884,9 @@ public class QuotePricingResult
 
     // Per-item results (same order as input items)
     public List<QuoteItemPricingResult> ItemResults { get; set; } = new();
+
+    // Pending Custom Powder Order preview — populated only when no "Custom Powder Order" item
+    // exists yet (first save scenario). Amount and color list let the UI show a preview row.
+    public decimal CustomPowderOrderAmount { get; set; }
+    public List<string> CustomPowderOrderColors { get; set; } = new();
 }