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/Jobs/Details.cshtml

@@ -326,6 +326,13 @@
                             <i class="bi bi-plus-circle me-1"></i>Add Item
                         </button>
                     </div>
+                @if (Model.Items.Any(i => i.Description != null && i.Description.StartsWith("Custom Powder Order")))
+                {
+                    <div class="alert alert-warning alert-permanent mx-3 mt-3 mb-0" role="alert">
+                        <i class="bi bi-truck me-2"></i><strong>Custom Powder Order</strong> &mdash;
+                        The line item below shows material cost only. Remember to add shipping before invoicing this job.
+                    </div>
+                }
                 @if (Model.Items.Any())
                 {
                     var allItems      = Model.Items.ToList();