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/Helpers/HelpKnowledgeBase.cs

@@ -302,7 +302,7 @@ public static class HelpKnowledgeBase
 
         **Changing the customer on a job:** On the Job Details page, the Customer field is an always-visible dropdown. Select a different customer — a confirmation banner appears. Click **Save** to apply or **Cancel** to revert. Use this to correct a misassigned job or to move a walk-in job to a customer's proper record after they've been added to the system.
 
-        **Creating an invoice from a job:** On the Job Details page, look for the Invoice section and click "Create Invoice."
+        **Creating an invoice from a job:** On the Job Details page, look for the Invoice section and click "Create Invoice." The system pre-fills all line items, pricing, discount, tax rate, payment terms, and due date from the job and customer automatically. Review the Totals panel on the right — if a discount was applied to the job it will show as a red "Discount Applied" line. Adjust anything you need, then save.
 
         **Work Order QR Codes:** Every printed job work order includes two tiers of QR codes — one for viewing the job, and a separate set for taking action on it. All QR codes require the worker to be logged in.
 
@@ -314,6 +314,8 @@ public static class HelpKnowledgeBase
 
         All QR codes require login — workers must have an active account. Logging in once on their phone is sufficient for the session.
 
+        **Logging material usage from a PC (without QR scan):** On the Job Details page, expand the Materials Used section and click **Log Material**. A modal opens where you can: select any inventory item from a dropdown (current stock level shown), choose whether to enter the amount used or the amount remaining (the system calculates usage automatically), pick a reason (Job Usage or Waste/Spillage), and add optional notes. Saves immediately and updates inventory on hand.
+
         **Blank Work Order:** Print a pre-formatted paper work order to hand to a walk-in customer before creating a digital job record.
         - Access: Jobs list page → printer icon button "Blank Work Order" in the top-right toolbar. Or navigate directly to /WorkOrder/Blank.
         - The PDF opens in a new tab ready to print. It includes: company logo and address, Drop Off Date field, Client Name / Client Phone / Due Date fields, 12-row parts table (Part Description / Color / Quote), Notes box, customizable Terms & Conditions text, and a Customer Signature line.