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.Web/Views/Quotes/Edit.cshtml

@@ -231,6 +231,28 @@
                     <p class="small">Click <strong>Add Item</strong> to get started.</p>
                 </div>
                 <div id="itemCardsContainer"></div>
+                <div id="customPowderOrderPreview" class="d-none mt-2">
+                    <div class="quote-item-card border-warning" style="border-style:dashed!important;background:var(--bs-warning-bg-subtle,#fff8e1);">
+                        <div class="d-flex align-items-start gap-2">
+                            <div class="flex-grow-1">
+                                <div class="d-flex align-items-center flex-wrap gap-1 mb-1">
+                                    <span class="badge bg-warning text-dark item-badge"><i class="bi bi-truck me-1"></i>Powder Order</span>
+                                    <span class="fw-semibold" id="customPowderOrderPreviewDesc">Custom Powder Order</span>
+                                </div>
+                                <div class="text-muted small"><span id="customPowderOrderPreviewPrice"></span> &mdash; edit total to include shipping</div>
+                            </div>
+                            <div class="text-center flex-shrink-0" style="min-width:45px;">1</div>
+                            <div class="flex-shrink-0" style="min-width:90px;">
+                                <input type="number" id="customPowderOrderPriceInput"
+                                       class="form-control form-control-sm text-end"
+                                       min="0" step="0.01" placeholder="0.00"
+                                       title="Enter total including any shipping"
+                                       oninput="onCustomPowderPriceEdit(this.value)">
+                            </div>
+                            <div style="min-width:66px;"></div>
+                        </div>
+                    </div>
+                </div>
 
                 <!-- Hidden fields written by wizard JS -->
                 <div id="hiddenFieldsContainer"></div>
@@ -500,6 +522,7 @@
         powderCostOverride   = item.PowderCostOverride,
         isGenericItem        = item.IsGenericItem,
         isLaborItem          = item.IsLaborItem,
+        isSalesItem          = item.IsSalesItem,
         isAiItem             = item.IsAiItem,
         isCustomFormulaItem  = item.IsCustomFormulaItem,
         customItemTemplateId = item.CustomItemTemplateId,
@@ -521,7 +544,7 @@
             sequence           = c.Sequence,
             inventoryItemId    = c.InventoryItemId,
             colorName          = c.ColorName,
-            vendorId           = c.VendorId,
+            supplierId         = c.VendorId,
             colorCode          = c.ColorCode,
             finish             = c.Finish,
             coverageSqFtPerLb  = c.CoverageSqFtPerLb,