Add Credit Memos standalone management module

CreditMemosController with Index, Details, Create, Apply, and Void actions.
All business logic (atomic apply transaction, RemainingBalance cap,
customer.CreditBalance adjustment, auto-Paid invoice when BalanceDue hits zero)
mirrors the invoice-centric IssueCreditMemo/ApplyCredit/VoidCreditMemo actions in
InvoicesController but redirects back to the credit memo rather than an invoice.

Views: Index (stats bar, status+search filter, table), Details (two-col layout
with application history table and Bootstrap Apply/Void confirm modals),
Create (customer dropdown, amount, reason, notes, optional expiry).

Apply modal populates amount automatically from min(remaining credit,
invoice balance due) via credit-memo.js data-attribute wiring (no inline scripts).

Nav: Credit Memos added to Billing & Payments section in _Layout.

Build: 0 errors. Unit tests: 200/200.

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

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -1077,6 +1077,10 @@
                             <span>Online Payments</span>
                         </a>
                     }
+                    <a asp-controller="CreditMemos" asp-action="Index" class="nav-link">
+                        <i class="bi bi-journal-minus"></i>
+                        <span>Credit Memos</span>
+                    </a>
                     <a asp-controller="GiftCertificates" asp-action="Index" class="nav-link">
                         <i class="bi bi-gift"></i>
                         <span>Gift Certificates</span>