Add loading spinner and disable button during file upload

sage_validation/file_validator/templates/upload.html

@@ -13,10 +13,21 @@
                        class="w-full border border-gray-300 p-3 rounded-lg focus:outline-none focus:ring focus:ring-blue-400">
             </div>
 
-            <button type="submit"
+            <button id="submitBtn" type="submit"
                     class="w-full bg-blue-600 text-white py-3 rounded-lg hover:bg-blue-700 focus:outline-none focus:ring focus:ring-blue-400">
                 Upload
             </button>
+
+            <!-- Loading Spinner -->
+            <div id="loadingSpinner" class="hidden flex justify-center">
+                <svg class="animate-spin h-6 w-6 text-blue-600" xmlns="http://www.w3.org/2000/svg" fill="none"
+                     viewBox="0 0 24 24">
+                    <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor"
+                            stroke-width="4"></circle>
+                    <path class="opacity-75" fill="currentColor"
+                          d="M4 12a8 8 0 018-8v4a4 4 0 00-4 4H4z"/>
+                </svg>
+            </div>
         </form>
 
         <!-- Error Display -->
@@ -45,6 +56,8 @@
             e.preventDefault();
 
             const fileInput = document.getElementById('fileInput');
+            const submitBtn = document.getElementById('submitBtn');
+            const loadingSpinner = document.getElementById('loadingSpinner');
             const errorSection = document.getElementById('errorSection');
             const errorList = document.getElementById('errorList');
             const successSection = document.getElementById('successSection');
@@ -58,6 +71,9 @@
             successSection.classList.add('hidden');
             downloadSection.classList.add('hidden');
 
+            submitBtn.disabled = true;
+            loadingSpinner.classList.remove('hidden');
+
             const formData = new FormData();
             formData.append('file', fileInput.files[0]);
 
@@ -105,6 +121,9 @@
                 li.textContent = 'Failed to connect to the server. Please check your internet connection.';
                 errorList.appendChild(li);
                 errorSection.classList.remove('hidden');
+            } finally {
+                loadingSpinner.classList.add('hidden');
+                submitBtn.disabled = false;
             }
         });
     </script>