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.Core/Entities/Company.cs

@@ -112,6 +112,12 @@ public class Company : BaseEntity
     /// </summary>
     public AccountingMethod AccountingMethod { get; set; } = AccountingMethod.Accrual;
 
+    /// <summary>
+    /// When set, prevents creating or editing accounting entries (JEs, bills, expenses) with dates
+    /// on or before this date. Protects closed periods from accidental backdating. Null = no lock.
+    /// </summary>
+    public DateTime? BookLockedThrough { get; set; }
+
     // Settings
     public string? TimeZone { get; set; } = "America/New_York";
     public byte[]? LogoData { get; set; }  // Legacy - kept for backward compatibility