feat: add vendor optional fields and configuration hooks

- Implemented VendorOptionalFields component to handle optional vendor details (mobile, email, address) with validation.
- Created useCustomerConfig hook to load optional customer field settings from Sales configuration.
- Developed useProcurementConfig hook to manage Procurement configuration flags.
- Introduced accessConfigApi for managing access configuration settings.
- Added openingBalancesApi for handling opening balances operations.
- Created procurementConfigApi for managing procurement configuration settings.
- Implemented nav-routes for permission-based routing.
- Developed PDF generation for purchase orders with dynamic vendor and item details.
- Added purchase-order tax helpers for tax calculations and formatting.
- Defined types for opening balances and related operations.

Backend/ERPCore/System/Errors/ErrorCodes.cs

@@ -87,6 +87,9 @@ public static class ErrorCodes
     // A submitted GL account mapping names an account GL does not have, or cannot post to.
     public const string GlAccountInvalid = "GL_ACCOUNT_INVALID";
 
+    // Opening balances: each kind can be posted only once.
+    public const string OpeningBalanceAlreadyPosted = "OPENING_BALANCE_ALREADY_POSTED";
+
     // Sales invoice payments
     public const string SalesInvoiceNotPayable = "SALES_INVOICE_NOT_PAYABLE";
     public const string SalesInvoicePaymentExceedsBalance = "SALES_INVOICE_PAYMENT_EXCEEDS_BALANCE";

Backend/ERPCore/System/Time/BusinessClock.cs

@@ -53,6 +53,10 @@ public static class BusinessClock
     /// <summary>Today's business day.</summary>
     public static DateOnly Today => DateOnly.FromDateTime(Now);
 
+    /// <summary>The UTC instant a business day starts at (its local midnight) — the inclusive lower bound of "that day".</summary>
+    public static DateTime StartOfDayUtc(DateOnly day) =>
+        TimeZoneInfo.ConvertTimeToUtc(day.ToDateTime(TimeOnly.MinValue, DateTimeKind.Unspecified), Zone);
+
     /// <summary>
     /// The business day a UTC instant falls on. Prefer this over <see cref="Today"/> when a
     /// flow already captured a <c>now</c>/<c>createdAt</c>: it keeps the period lookup and the