Phase D: Add Vendor Credits (AP cycle completion)

- VendorCredit, VendorCreditLineItem, VendorCreditApplication entities
- VendorCreditStatus enum (Open, PartiallyApplied, Applied, Voided)
- Migration AddVendorCredits: three new tables
- IUnitOfWork/UnitOfWork wired with all three repositories
- VendorCreditsController: Index (status tabs), Create, Details, Post, Apply, Void
- Post action: DR AP, CR each expense line (reverses original expense)
- Apply action: links credit to bill, updates Bill.AmountPaid and bill status
- Views: Index (summary cards + table), Create (dynamic line grid), Details (apply panel)
- Nav: Vendor Credits added to Finance section in _Layout

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

src/PowderCoating.Core/Interfaces/IUnitOfWork.cs

@@ -95,6 +95,11 @@ public interface IUnitOfWork : IDisposable
     IRepository<JournalEntry> JournalEntries { get; }
     IRepository<JournalEntryLine> JournalEntryLines { get; }
 
+    // Vendor Credits
+    IRepository<VendorCredit> VendorCredits { get; }
+    IRepository<VendorCreditLineItem> VendorCreditLineItems { get; }
+    IRepository<VendorCreditApplication> VendorCreditApplications { get; }
+
     // Notifications — typed repository for IgnoreQueryFilters-based history lookups
     INotificationLogRepository NotificationLogs { get; }
     IRepository<NotificationTemplate> NotificationTemplates { get; }