Add formula template export/import and unsaved-changes guard

- Export: GET /CompanySettings/ExportCustomItemTemplates downloads all
  company templates as an indented JSON backup (strips internal IDs/paths)
- Import: POST /CompanySettings/ImportCustomItemTemplates restores from
  that file; runs full field + formula validation, skips name duplicates,
  returns per-item results (imported / skipped / errors)
- Unsaved-changes guard: cfModal now intercepts backdrop/ESC/X when the
  form is dirty and prompts before discarding work
- Export and Import buttons added to the Custom Formulas card header

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/CompanySettings/Index.cshtml

@@ -2197,6 +2197,15 @@
                         <button type="button" class="btn btn-outline-secondary btn-sm" onclick="cfShowWalkthrough()">
                             <i class="bi bi-question-circle me-1"></i>How it works
                         </button>
+                        <a href="/CompanySettings/ExportCustomItemTemplates"
+                           class="btn btn-outline-secondary btn-sm"
+                           title="Download all templates as a JSON backup file">
+                            <i class="bi bi-download me-1"></i>Export
+                        </a>
+                        <button type="button" class="btn btn-outline-secondary btn-sm" onclick="cfShowImport()"
+                                title="Restore templates from a JSON backup file">
+                            <i class="bi bi-upload me-1"></i>Import
+                        </button>
                         <button type="button" class="btn btn-primary btn-sm" onclick="cfShowCreate()">
                             <i class="bi bi-plus-circle me-1"></i>New Template
                         </button>
@@ -2281,6 +2290,40 @@
     </div>
 </div>
 
+<!-- Custom Formula Import Modal -->
+<div class="modal fade" id="cfImportModal" tabindex="-1" aria-labelledby="cfImportModalLabel" aria-hidden="true">
+    <div class="modal-dialog">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title" id="cfImportModalLabel">
+                    <i class="bi bi-upload me-2"></i>Import Formula Templates
+                </h5>
+                <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+            </div>
+            <div class="modal-body">
+                <p class="text-muted small mb-3">
+                    Select a <code>.json</code> file previously exported from this page.
+                    Templates whose name already exists in your account will be skipped.
+                </p>
+                <div class="mb-3">
+                    <label class="form-label fw-semibold">Backup file <span class="text-danger">*</span></label>
+                    <input type="file" id="cfImportFile" class="form-control" accept=".json" />
+                </div>
+                <div id="cfImportResults" class="d-none">
+                    <hr />
+                    <div id="cfImportSummary"></div>
+                </div>
+            </div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
+                <button type="button" class="btn btn-primary" id="cfImportBtn" onclick="cfSubmitImport()">
+                    <i class="bi bi-upload me-1"></i>Import
+                </button>
+            </div>
+        </div>
+    </div>
+</div>
+
 <!-- Custom Formula Walkthrough Modal -->
 <div class="modal fade" id="cfWalkthroughModal" tabindex="-1" aria-labelledby="cfWalkthroughLabel" aria-hidden="true">
     <div class="modal-dialog modal-lg modal-dialog-centered modal-dialog-scrollable">