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/BillsController.cs

@@ -321,6 +321,19 @@ public class BillsController : Controller
         try
         {
             var currentUser = await _userManager.GetUserAsync(User);
+
+            // Period lock check — block if the bill date is in a locked period
+            if (currentUser != null)
+            {
+                var co = await _unitOfWork.Companies.GetByIdAsync(currentUser.CompanyId);
+                if (Web.Helpers.AccountingPeriodValidator.IsLocked(dto.BillDate, co?.BookLockedThrough))
+                {
+                    ModelState.AddModelError("BillDate", Web.Helpers.AccountingPeriodValidator.LockedMessage(co!.BookLockedThrough));
+                    await PopulateDropdownsAsync();
+                    return View(dto);
+                }
+            }
+
             Bill? bill = null;
 
             // Bill entity, PO back-reference, and optional immediate payment all commit