docs: add note about JSON re-import to export modal

Added styled information box explaining that only JSON exports can be
re-imported to continue workflows. Helps users understand format choices.

Author: JasonWarrenUK

src/lib/components/theia/ExportConfigModal.svelte

@@ -256,6 +256,9 @@
             📄 PDF <span class="badge">Soon</span>
           </button>
         </div>
+        <p class="help-text format-note">
+          <strong>Note:</strong> Only JSON exports can be re-imported to continue your workflow. Use Markdown or HTML for sharing and documentation.
+        </p>
       </div>
 
       <!-- Options -->
@@ -524,6 +527,18 @@
     margin-top: 0.25rem;
   }
 
+  .format-note {
+    margin-top: 0.75rem;
+    padding: 0.75rem;
+    background: #f8f9fa;
+    border-left: 3px solid #0d6efd;
+    border-radius: 4px;
+  }
+
+  .format-note strong {
+    color: #495057;
+  }
+
   .format-tabs {
     display: flex;
     gap: 0.5rem;