Make cash refunds reverse the sale instead of re-opening AR

Audit finding 9a: a cash refund posted DR AR (control up) while the customer
subledger went down — opposite directions, guaranteeing AR drift. Per the chosen
model, a cash refund now *reverses the sale*: DR Sales Returns (revenue portion)
+ DR Sales Tax Payable (tax portion) / CR Bank, with the customer AR balance left
untouched (the invoice stays paid; the sale is contra'd).

- New 4960 "Sales Returns & Allowances" contra-revenue account (seed + self-heal).
- Tax/revenue split centralized in Core RefundAllocation.Split so the posting and
  both reporting recomputes compute it identically (a mismatch would unbalance the
  trial balance).
- InvoicesController IssueRefund/CancelRefund: new posting + reversal; no AR/
  customer-balance change.
- LedgerService (per-account + prior balance): refunds debit Sales Returns + Sales
  Tax, no longer debit AR; bank credit unchanged.
- FinancialReportService: trial balance, balance sheet (retained earnings + tax
  liability), and P&L (contra-revenue line) all updated to match. Store-credit
  refunds are excluded everywhere (they post via CreditMemo, not the GL — 9b).

Verified: $108 refund (incl. $8 tax) -> bank -100... checks: assets -108 =
liabilities -8 + equity -100. New tests cover the split invariant and the ledger
postings. Build clean; 283 unit tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/PowderCoating.UnitTests/RefundAllocationTests.cs

@@ -0,0 +1,22 @@
+using PowderCoating.Core.Accounting;
+
+namespace PowderCoating.UnitTests;
+
+public class RefundAllocationTests
+{
+    [Theory]
+    [InlineData(108, 8, 108, 100, 8)]   // full refund, proportional tax
+    [InlineData(54, 8, 108, 50, 4)]     // half refund → half the tax
+    [InlineData(100, 0, 100, 100, 0)]   // no tax on the invoice
+    [InlineData(100, 8, 0, 100, 0)]     // zero invoice total → all returns, no tax
+    public void Split_AllocatesTaxProportionally_AndAlwaysSumsToRefund(
+        decimal amount, decimal invoiceTax, decimal invoiceTotal, decimal expectedReturns, decimal expectedTax)
+    {
+        var (returns, tax) = RefundAllocation.Split(amount, invoiceTax, invoiceTotal);
+
+        Assert.Equal(expectedReturns, returns);
+        Assert.Equal(expectedTax, tax);
+        // Invariant: the two portions must always reconstruct the refund exactly (no penny lost).
+        Assert.Equal(amount, returns + tax);
+    }
+}