Add GL account configuration and role mapping entities

- Introduced GlAccountConfig entity for mapping accounting roles to GL account codes.
- Added GlAccountConfigConfiguration for database configuration of the singleton GL account mapping.
- Created GlAccountRole enum to define accounting roles used in GL postings.
- Enhanced error handling with new error codes for unconfigured and invalid GL accounts.
- Updated existing DbContext to include GlAccountConfig DbSet.

Backend/ERPCore/Domain/Entities/GlAccountConfig.cs

@@ -0,0 +1,71 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Role → GL account code mapping — a singleton row, same shape as <see cref="GrnConfig"/>.
+/// <para>
+/// This is master data, not deployment config. It used to live in <c>appsettings.json</c> as
+/// per-module <c>Gl*Code</c> keys, which meant (a) only a redeploy could correct a mapping, and
+/// (b) nothing checked the codes existed in GL until something tried to post. Held here it is
+/// validated against GL's live chart of accounts on every save, so an unknown or non-postable
+/// account cannot be stored in the first place — see <c>GlAccountConfigService.UpdateAsync</c>.
+/// </para>
+/// <para>
+/// Codes are nullable: a fresh install has nothing mapped, and a posting flow that needs an
+/// unmapped role fails with <c>GL_ACCOUNT_NOT_CONFIGURED</c> naming that role. Width matches
+/// GL's own <c>accounts.account_code varchar(20)</c>.
+/// </para>
+/// </summary>
+public class GlAccountConfig
+{
+    /// <summary>Always 1 - the singleton row's id.</summary>
+    public const int SingletonId = 1;
+
+    /// <summary>Matches GL's <c>accounts.account_code</c> column width.</summary>
+    public const int CodeMaxLength = 20;
+
+    public int ConfigId { get; set; }
+
+    /// <summary>Stock on hand (was <c>Grn|Adjustment|SalesDayEnd|Fifo:GlInventoryAccountCode</c>).</summary>
+    public string? InventoryAccountCode { get; set; }
+
+    /// <summary>Recoverable input VAT (was <c>Grn:GlVatRecoverableAccountCode</c>).</summary>
+    public string? VatRecoverableAccountCode { get; set; }
+
+    /// <summary>Trade debtors (was <c>SalesDayEnd|SalesInvoice:GlAccountsReceivableCode</c>).</summary>
+    public string? AccountsReceivableCode { get; set; }
+
+    /// <summary>Goods received not invoiced (was <c>Grn|PurchaseInvoice|Fifo:GlClearingAccountCode</c>).</summary>
+    public string? ClearingAccountCode { get; set; }
+
+    /// <summary>Output tax payable (was <c>SalesDayEnd:GlTaxPayableAccountCode</c>).</summary>
+    public string? TaxPayableAccountCode { get; set; }
+
+    /// <summary>Trade creditors (was <c>PurchaseInvoice:GlAccountsPayableCode</c>).</summary>
+    public string? AccountsPayableCode { get; set; }
+
+    /// <summary>Sales revenue (was <c>SalesDayEnd:GlSalesRevenueAccountCode</c>).</summary>
+    public string? SalesRevenueAccountCode { get; set; }
+
+    /// <summary>Sales discount given (was <c>SalesDayEnd:GlSalesDiscountAccountCode</c>).</summary>
+    public string? SalesDiscountAccountCode { get; set; }
+
+    /// <summary>Inventory gain on adjustment/count (was <c>Adjustment:GlGainAccountCode</c>).</summary>
+    public string? InventoryGainAccountCode { get; set; }
+
+    /// <summary>Cost of goods sold (was <c>SalesDayEnd:GlCogsAccountCode</c>).</summary>
+    public string? CogsAccountCode { get; set; }
+
+    /// <summary>Inventory loss on adjustment/count (was <c>Adjustment:GlLossAccountCode</c>).</summary>
+    public string? InventoryLossAccountCode { get; set; }
+
+    /// <summary>Cash takings at day-end (was <c>SalesDayEnd:GlCashAccountCode</c>).</summary>
+    public string? CashAccountCode { get; set; }
+
+    public DateTime? UpdatedAt { get; set; }
+
+    public int? UpdatedBy { get; set; }
+    public User? UpdatedByUser { get; set; }
+
+    /// <summary>Optimistic concurrency token (ETag source); bumped by ErpDbContext.BumpRowVersions.</summary>
+    public uint RowVersion { get; set; }
+}