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

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>