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.Web/Views/Shared/_Layout.cshtml

@@ -1134,6 +1134,10 @@
                             <i class="bi bi-journal-text"></i>
                             <span>Journal Entries</span>
                         </a>
+                        <a asp-controller="VendorCredits" asp-action="Index" class="nav-link">
+                            <i class="bi bi-arrow-return-left"></i>
+                            <span>Vendor Credits</span>
+                        </a>
                         if (hasReports)
                         {
                             <a asp-controller="AccountingExport" asp-action="Index" class="nav-link">