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/CompanySettingsController.cs

@@ -160,6 +160,10 @@ public class CompanySettingsController : Controller
                     UpdatedAt = t.UpdatedAt
                 }).ToList();
 
+            ViewBag.BookLockedThrough = company.BookLockedThrough.HasValue
+                ? (DateTime?)company.BookLockedThrough.Value.ToLocalTime()
+                : null;
+
             return View(dto);
         }
         catch (FormatException fex)
@@ -227,6 +231,34 @@ public class CompanySettingsController : Controller
         }
     }
 
+    /// <summary>
+    /// Locks the books through the given date, preventing new or edited accounting entries
+    /// (JEs, bills, expenses) from being dated on or before this date. Null clears the lock.
+    /// </summary>
+    [HttpPost]
+    [ValidateAntiForgeryToken]
+    public async Task<IActionResult> SetPeriodLock(DateTime? lockThrough)
+    {
+        var companyId = _tenantContext.GetCurrentCompanyId();
+        if (companyId == null) return BadRequest();
+
+        var company = await _unitOfWork.Companies.GetByIdAsync(companyId.Value);
+        if (company == null) return NotFound();
+
+        company.BookLockedThrough = lockThrough.HasValue
+            ? DateTime.SpecifyKind(lockThrough.Value.Date, DateTimeKind.Utc)
+            : null;
+        company.UpdatedAt = DateTime.UtcNow;
+        await _unitOfWork.Companies.UpdateAsync(company);
+        await _unitOfWork.CompleteAsync();
+
+        TempData["Success"] = lockThrough.HasValue
+            ? $"Books locked through {lockThrough.Value:MMMM d, yyyy}."
+            : "Period lock cleared — all periods are now open.";
+
+        return RedirectToAction(nameof(Index), null, "company-info");
+    }
+
     /// <summary>
     /// Serves the current company's logo as a binary file response. Logos are stored on the filesystem
     /// via <see cref="ICompanyLogoService"/> (primary) or as raw bytes in <c>Company.LogoData</c>