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.Core/Interfaces/IUnitOfWork.cs

@@ -43,6 +43,7 @@ public interface IUnitOfWork : IDisposable
     IJobPhotoRepository JobPhotos { get; }
     IRepository<JobNote> JobNotes { get; }
     IRepository<CustomerNote> CustomerNotes { get; }
+    IRepository<CustomerContact> CustomerContacts { get; }
     IRepository<CustomerPreferredPowder> CustomerPreferredPowders { get; }
     IRepository<JobStatusHistory> JobStatusHistory { get; }
     IRepository<PricingTier> PricingTiers { get; }