Add company default GL accounts; move QB sign-fix to platform

Default accounts: companies can now set a default Revenue, COGS, and
Inventory account (Chart of Accounts -> "Default Accounts" card). Stored
on CompanyPreferences (3 nullable FKs + migration). Used as the fallback
when an item or invoice line leaves its account blank:
 - Invoice lines fall back to the default Revenue account, then to 4000.
 - New inventory/catalog items are pre-filled with the COGS/Inventory
   defaults, so the value is stored on the item and both live posting and
   the balance-recompute path stay consistent.
Blank defaults = unchanged behavior, so nothing changes until a company
opts in. Setting both COGS + Inventory enables perpetual-inventory COGS
posting (warned in the UI and help docs). Help KB + Settings article
updated.

Also moves the "Fix QB Import Signs" tool off the company Chart of
Accounts page (was CompanyAdmin-visible) to the SuperAdmin-only platform
Company Details page, operating on the target company.

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -275,10 +275,18 @@ public class InventoryController : Controller
         ViewBag.UseMetric = useMetric;
         ViewBag.CoverageUnit = _measurementService.GetCoverageUnitLabel(useMetric);
 
+        // Pre-fill the GL account dropdowns from the company's configured defaults so new items
+        // inherit them (the user can still change or clear them on the form).
+        var companyId = _tenantContext.GetCurrentCompanyId() ?? 0;
+        var prefs = await _unitOfWork.CompanyPreferences
+            .FirstOrDefaultAsync(p => p.CompanyId == companyId && !p.IsDeleted);
+
         return View(new CreateInventoryItemDto
         {
             CoverageSqFtPerLb = 30,
-            TransferEfficiency = 65
+            TransferEfficiency = 65,
+            InventoryAccountId = prefs?.DefaultInventoryAccountId,
+            CogsAccountId = prefs?.DefaultCogsAccountId
         });
     }