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.Infrastructure/Repositories/UnitOfWork.cs

@@ -159,6 +159,8 @@ public class UnitOfWork : IUnitOfWork
 
     // Recurring Transactions
     private IRepository<RecurringTemplate>? _recurringTemplates;
+    private IRepository<FixedAsset>? _fixedAssets;
+    private IRepository<FixedAssetDepreciationEntry>? _fixedAssetDepreciationEntries;
 
     /// <summary>
     /// Initialises the unit of work with the scoped <paramref name="context"/>.
@@ -567,6 +569,10 @@ public class UnitOfWork : IUnitOfWork
     /// <summary>Repository for <see cref="RecurringTemplate"/> — saved recipes that auto-generate bills or expenses on a schedule.</summary>
     public IRepository<RecurringTemplate> RecurringTemplates =>
         _recurringTemplates ??= new Repository<RecurringTemplate>(_context);
+    public IRepository<FixedAsset> FixedAssets =>
+        _fixedAssets ??= new Repository<FixedAsset>(_context);
+    public IRepository<FixedAssetDepreciationEntry> FixedAssetDepreciationEntries =>
+        _fixedAssetDepreciationEntries ??= new Repository<FixedAssetDepreciationEntry>(_context);
 
     /// <summary>
     /// Flushes all pending changes in the EF Core change tracker to the database.