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/Help/Jobs.cshtml

@@ -273,6 +273,29 @@
                 appear as sub-lines under the item on the job details page.
             </p>
 
+            <h3 class="h6 fw-semibold mt-4 mb-2"><i class="bi bi-truck me-1 text-warning"></i>Custom Powder Order</h3>
+            <p>
+                When a coat uses a <strong>custom powder</strong> (manually entered cost per lb, no inventory item
+                selected) or an <strong>incoming powder</strong> (a color being ordered that is not yet in stock),
+                the material cost is separated from the item price and collected into a single
+                <strong>Custom Powder Order</strong> line item. This keeps per-item pricing clean and lets you
+                add shipping or freight before the customer sees the total.
+            </p>
+            <ul class="mb-3">
+                <li class="mb-2">
+                    While building the job, a dashed yellow <strong>Powder Order</strong> preview card appears
+                    below the item cards. Edit the price to include any shipping before saving.
+                </li>
+                <li class="mb-2">
+                    On the saved job, the Custom Powder Order appears as its own line item with the ordered
+                    color name(s) in its description.
+                </li>
+                <li>
+                    The Custom Powder Order is created once on first save. The price is user-owned after that
+                    and will not be overwritten by the system on subsequent saves.
+                </li>
+            </ul>
+
             <h3 class="h6 fw-semibold mt-4 mb-2">Save to Product Catalog</h3>
             <p>
                 After completing the coatings and prep services steps, <strong>Calculated</strong> and