Update AI assistant and help docs for invoice and material logging changes

- HelpKnowledgeBase: invoice-from-job now mentions discount carried over,
  Discount Applied display row, and negative line items; new entry for
  PC-based Log Material modal on job details
- Help/Invoices.cshtml: from-job steps updated with discount/terms/due date
  pre-fill detail; sending section corrects due date source (quote/customer)
- Help/Jobs.cshtml: new "Logging Material Usage from a PC" section documenting
  the Log Material modal alongside the existing QR scan instructions

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

src/PowderCoating.Web/Views/Help/Invoices.cshtml

@@ -48,8 +48,9 @@
             <ol class="mb-3">
                 <li class="mb-2">Open the job from <strong>Operations &rsaquo; Jobs</strong> and go to its Details page.</li>
                 <li class="mb-2">Scroll to the <strong>Invoice</strong> section near the bottom of the page.</li>
-                <li class="mb-2">Click <strong>Create Invoice</strong>. The system generates an invoice pre-filled with all the job's line items and the final pricing.</li>
-                <li class="mb-2">Review the invoice — check line items, totals, and the due date — then click <strong>Save Invoice</strong>.</li>
+                <li class="mb-2">Click <strong>Create Invoice</strong>. The system pre-fills all line items, the discount, tax rate, payment terms, and due date from the job and customer automatically.</li>
+                <li class="mb-2">Review the <strong>Totals</strong> panel on the right &mdash; if a discount was applied to the job it shows as a red <em>Discount Applied</em> line below the subtotal. Negative line items are allowed if you need to apply a manual credit or price adjustment.</li>
+                <li class="mb-2">Adjust anything you need, then click <strong>Save Invoice</strong>.</li>
             </ol>
 
             <h3 class="h6 fw-semibold mt-3 mb-2">From the Invoices list (manual)</h3>
@@ -139,7 +140,7 @@
                 <li class="mb-2">Open the invoice from <strong>Operations &rsaquo; Invoices</strong> or from the job's Details page.</li>
                 <li class="mb-2">Click <strong>Send Invoice</strong>. The status changes from Draft to Sent.</li>
                 <li class="mb-2">If email notifications are configured, the customer receives an email with the invoice details and total due.</li>
-                <li class="mb-2">A due date is set automatically based on the customer's payment terms (e.g., Net 30 means the due date is 30 days from today).</li>
+                <li class="mb-2">The due date and payment terms are pre-filled from the source quote (if the job came from a quote) or the customer&rsquo;s payment terms &mdash; you can always override them before saving.</li>
             </ol>
             <p>
                 You can also click <strong>Download PDF</strong> on any invoice to generate a print-ready PDF