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.Infrastructure/Repositories/UnitOfWork.cs

@@ -70,6 +70,7 @@ public class UnitOfWork : IUnitOfWork
     private IJobPhotoRepository? _jobPhotos;
     private IRepository<JobNote>? _jobNotes;
     private IRepository<CustomerNote>? _customerNotes;
+    private IRepository<CustomerContact>? _customerContacts;
     private IRepository<CustomerPreferredPowder>? _customerPreferredPowders;
     private IRepository<JobStatusHistory>? _jobStatusHistory;
     private IRepository<PricingTier>? _pricingTiers;
@@ -322,6 +323,9 @@ public class UnitOfWork : IUnitOfWork
     /// <summary>Repository for <see cref="CustomerNote"/> free-text staff notes on customer records; tenant-filtered with soft delete.</summary>
     public IRepository<CustomerNote> CustomerNotes =>
         _customerNotes ??= new Repository<CustomerNote>(_context);
+    /// <summary>Repository for <see cref="CustomerContact"/> additional contacts (billing, ops, drop-off) on commercial accounts; tenant-filtered with soft delete.</summary>
+    public IRepository<CustomerContact> CustomerContacts =>
+        _customerContacts ??= new Repository<CustomerContact>(_context);
     public IRepository<CustomerPreferredPowder> CustomerPreferredPowders =>
         _customerPreferredPowders ??= new Repository<CustomerPreferredPowder>(_context);