Add CRM features: Additional Contacts, Lead Source, Ship-To Address; update Help docs

- New CustomerContact entity + migration (AddCustomerContactsAndCrmFields)
- Customer.LeadSource + ShipToAddress/City/State/ZipCode/Country fields
- Additional Contacts card on Customer Details with AJAX add/edit/delete
- Lead Source dropdown on Create/Edit; Ship-To section on Create/Edit
- Customer Details: side-by-side billing/ship-to when ship-to is set
- Help docs: Customers (contacts, ship-to, lead source, preferred powders, outstanding pickups)
- Help docs: Jobs (clone job, project name), Quotes (project name), Invoices (project name), Inventory (low stock clickable filter)
- HelpKnowledgeBase.cs updated for all features above

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

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

@@ -74,6 +74,7 @@
                 </li>
                 <li class="mb-2">Set the <strong>Quote Date</strong> (defaults to today) and the <strong>Expiry Date</strong> (defaults to the system's configured validity period).</li>
                 <li class="mb-2">Add a <strong>Subject</strong> or description to identify the work being quoted.</li>
+                <li class="mb-2">Optionally enter a <strong>Project Name</strong> &mdash; a short label that groups related work (e.g., &ldquo;Fleet Refresh Q2&rdquo;). It appears on the quote PDF and carries over to the job and invoice when converted.</li>
                 <li class="mb-2">Add one or more <strong>Line Items</strong> &mdash; see the Quote Items section below for item types.</li>
                 <li class="mb-2">Add any <strong>Notes</strong> for the customer (these appear on the printed quote).</li>
                 <li class="mb-2">Add any internal <strong>Notes</strong> that are for your team only.</li>