Phase F: Add Invoice Write-Off, Fixed Assets, Period Locking, and 1099 Tracking

- Invoice Write-Off: WriteOff POST action in InvoicesController posts bad-debt JE
  (DR bad debt expense / CR AR), reduces customer balance, marks invoice WrittenOff;
  write-off modal added to Invoice Details view with expense account selector
- Fixed Assets: FixedAsset + FixedAssetDepreciationEntry entities with straight-line
  depreciation; FixedAssetsController (Index/Create/Edit/Details/PostDepreciation/Delete);
  PostDepreciation auto-generates one JE per asset per period, skips already-posted,
  fully-depreciated, and disposed assets; full CRUD views + nav link
- Period Locking: Company.BookLockedThrough field; AccountingPeriodValidator static helper;
  lock check added to JE Post and Bill Create (blocks backdating into closed periods);
  SetPeriodLock action + date picker UI in Company Settings Accounting section
- 1099 Tracking: Is1099Vendor flag on Vendor entity + DTOs; checkbox in Create/Edit views;
  TaxReporting1099 report action + view lists payments by year, flags vendors >= $600;
  report card added to Reports Landing
- Migration AddFixedAssetsLockAnd1099 applied

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

src/PowderCoating.Web/Controllers/JournalEntriesController.cs

@@ -167,6 +167,14 @@ public class JournalEntriesController : Controller
             return RedirectToAction(nameof(Details), new { id });
         }
 
+        // Period lock check — block posting if the entry date falls in a locked period
+        var company = await _unitOfWork.Companies.GetByIdAsync(entry.CompanyId);
+        if (Web.Helpers.AccountingPeriodValidator.IsLocked(entry.EntryDate, company?.BookLockedThrough))
+        {
+            TempData["Error"] = Web.Helpers.AccountingPeriodValidator.LockedMessage(company!.BookLockedThrough);
+            return RedirectToAction(nameof(Details), new { id });
+        }
+
         await _unitOfWork.ExecuteInTransactionAsync(async () =>
         {
             entry.Status   = JournalEntryStatus.Posted;