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/Views/Vendors/Create.cshtml

@@ -186,6 +186,11 @@
                                     <input asp-for="IsActive" class="form-check-input" type="checkbox" checked />
                                     <label asp-for="IsActive" class="form-check-label">Active Vendor</label>
                                 </div>
+                                <div class="form-check form-switch mt-2">
+                                    <input asp-for="Is1099Vendor" class="form-check-input" type="checkbox" />
+                                    <label asp-for="Is1099Vendor" class="form-check-label">1099 Vendor</label>
+                                </div>
+                                <div class="form-text">Check if this vendor requires a 1099-NEC at year end (typically non-incorporated service providers paid ≥ $600).</div>
                             </div>
                         </div>
                     </div>

src/PowderCoating.Web/Views/Vendors/Edit.cshtml

@@ -188,6 +188,13 @@
                                 </div>
                                 <span asp-validation-for="CreditLimit" class="text-danger"></span>
                             </div>
+                            <div class="col-md-6">
+                                <div class="form-check form-switch mt-4">
+                                    <input asp-for="Is1099Vendor" class="form-check-input" type="checkbox" />
+                                    <label asp-for="Is1099Vendor" class="form-check-label">1099 Vendor</label>
+                                </div>
+                                <div class="form-text">Check if this vendor requires a 1099-NEC at year end (typically non-incorporated service providers paid ≥ $600).</div>
+                            </div>
                         </div>
                     </div>