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/Invoices/Details.cshtml

@@ -679,6 +679,13 @@
                                     </button>
                                 </form>
                             }
+                            @if (!isVoided && Model.BalanceDue > 0)
+                            {
+                                <button type="button" class="btn btn-outline-danger w-100"
+                                        data-bs-toggle="modal" data-bs-target="#writeOffModal">
+                                    <i class="bi bi-journal-x me-2"></i>Write Off
+                                </button>
+                            }
                             @if (isDraft)
                             {
                                 <form asp-action="Delete" asp-route-id="@Model.Id" method="post"
@@ -1308,6 +1315,55 @@
     </div>
 }
 
+<!-- Write-Off Modal -->
+@if (!isVoided && Model.BalanceDue > 0)
+{
+<div class="modal fade" id="writeOffModal" tabindex="-1">
+    <div class="modal-dialog">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title"><i class="bi bi-journal-x me-2 text-danger"></i>Write Off Invoice</h5>
+                <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+            </div>
+            <form asp-action="WriteOff" asp-route-id="@Model.Id" method="post">
+                @Html.AntiForgeryToken()
+                <div class="modal-body">
+                    <div class="alert alert-warning py-2 mb-3">
+                        <i class="bi bi-exclamation-triangle me-2"></i>
+                        This will write off the remaining balance of <strong>@Model.BalanceDue.ToString("C")</strong>
+                        as bad debt. A GL journal entry will be posted. This action cannot be undone.
+                    </div>
+                    <div class="mb-3">
+                        <label class="form-label">Bad Debt Expense Account</label>
+                        <select name="expenseAccountId" class="form-select">
+                            <option value="">— Use default bad debt account —</option>
+                            @if (ViewBag.ExpenseAccounts != null)
+                            {
+                                @foreach (var item in (IEnumerable<Microsoft.AspNetCore.Mvc.Rendering.SelectListItem>)ViewBag.ExpenseAccounts)
+                                {
+                                    <option value="@item.Value">@item.Text</option>
+                                }
+                            }
+                        </select>
+                        <div class="form-text">If blank, the system selects the first account with "bad" or "debt" in the name, or falls back to the first expense account.</div>
+                    </div>
+                    <div class="mb-3">
+                        <label class="form-label">Notes</label>
+                        <textarea name="notes" class="form-control" rows="2" placeholder="Reason for write-off (optional)"></textarea>
+                    </div>
+                </div>
+                <div class="modal-footer">
+                    <button type="button" class="btn btn-outline-secondary" data-bs-dismiss="modal">Cancel</button>
+                    <button type="submit" class="btn btn-danger">
+                        <i class="bi bi-journal-x me-2"></i>Write Off @Model.BalanceDue.ToString("C")
+                    </button>
+                </div>
+            </form>
+        </div>
+    </div>
+</div>
+}
+
 @section Scripts {
 <script>
     function openEditPaymentModal(paymentId, invoiceId, paymentDate, paymentMethod, reference, notes, depositAccountId) {