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/Jobs.cshtml

@@ -607,13 +607,28 @@
                 no anonymous bumps.
             </p>
 
-            <h3 class="h6 fw-semibold mt-3 mb-2"><i class="bi bi-box-seam me-1"></i>Bottom QR — Log Powder Usage</h3>
+            <h3 class="h6 fw-semibold mt-3 mb-2"><i class="bi bi-box-seam me-1"></i>Bottom QR &mdash; Log Powder Usage</h3>
             <p>
                 One QR per unique powder on the job. Scanning opens the inventory usage log page pre-filled
                 with that powder and the job number, so you can record actual lbs used in seconds without
                 navigating through the app.
             </p>
 
+            <h3 class="h6 fw-semibold mt-3 mb-2"><i class="bi bi-droplet-half me-1"></i>Logging Material Usage from a PC</h3>
+            <p>
+                You don&rsquo;t need a phone or QR code to log material usage. On the Job Details page, expand the
+                <strong>Materials Used</strong> section and click <strong>Log Material</strong>. A modal opens where you can:
+            </p>
+            <ul class="mb-2">
+                <li>Select any inventory item from a searchable dropdown &mdash; the item&rsquo;s current stock level is shown when you pick it.</li>
+                <li>Choose <strong>Amount Used</strong> (enter how much was consumed) or <strong>Amount Remaining</strong> (enter what&rsquo;s left in the bag &mdash; the system calculates the usage automatically).</li>
+                <li>Pick a reason: <em>Job Usage</em> or <em>Waste / Spillage</em>.</li>
+                <li>Add optional notes.</li>
+            </ul>
+            <p>
+                Saving immediately reduces the item&rsquo;s stock on hand and creates an entry in the Inventory Activity ledger, exactly like a QR scan would. The QR scan icon is still available next to the button for mobile workers.
+            </p>
+
             <div class="alert alert-permanent alert-info d-flex gap-2 mb-0" role="alert">
                 <i class="bi bi-lock flex-shrink-0 mt-1"></i>
                 <div>