Feature/toasts and error handling (#4496)

# Description of Changes

- Added error handling and toast notifications

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/core/src/main/java/stirling/software/SPDF/controller/api/MergeController.java

@@ -3,6 +3,7 @@ package stirling.software.SPDF.controller.api;
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Paths;
 import java.nio.file.attribute.BasicFileAttributes;
@@ -20,6 +21,8 @@ import org.apache.pdfbox.pdmodel.interactive.documentnavigation.outline.PDOutlin
 import org.apache.pdfbox.pdmodel.interactive.form.PDAcroForm;
 import org.apache.pdfbox.pdmodel.interactive.form.PDField;
 import org.apache.pdfbox.pdmodel.interactive.form.PDSignatureField;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.multipart.MultipartFile;
@@ -111,6 +114,32 @@ public class MergeController {
         }
     }
 
+    // Parse client file IDs from JSON string
+    private String[] parseClientFileIds(String clientFileIds) {
+        if (clientFileIds == null || clientFileIds.trim().isEmpty()) {
+            return new String[0];
+        }
+        try {
+            // Simple JSON array parsing - remove brackets and split by comma
+            String trimmed = clientFileIds.trim();
+            if (trimmed.startsWith("[") && trimmed.endsWith("]")) {
+                String inside = trimmed.substring(1, trimmed.length() - 1).trim();
+                if (inside.isEmpty()) {
+                    return new String[0];
+                }
+                String[] parts = inside.split(",");
+                String[] result = new String[parts.length];
+                for (int i = 0; i < parts.length; i++) {
+                    result[i] = parts[i].trim().replaceAll("^\"|\"$", "");
+                }
+                return result;
+            }
+        } catch (Exception e) {
+            log.warn("Failed to parse client file IDs: {}", clientFileIds, e);
+        }
+        return new String[0];
+    }
+
     // Adds a table of contents to the merged document using filenames as chapter titles
     private void addTableOfContents(PDDocument mergedDocument, MultipartFile[] files) {
         // Create the document outline
@@ -177,15 +206,48 @@ public class MergeController {
 
             PDFMergerUtility mergerUtility = new PDFMergerUtility();
             long totalSize = 0;
-            for (MultipartFile multipartFile : files) {
+            List<Integer> invalidIndexes = new ArrayList<>();
+            for (int index = 0; index < files.length; index++) {
+                MultipartFile multipartFile = files[index];
                 totalSize += multipartFile.getSize();
                 File tempFile =
                         GeneralUtils.convertMultipartFileToFile(
                                 multipartFile); // Convert MultipartFile to File
                 filesToDelete.add(tempFile); // Add temp file to the list for later deletion
+
+                // Pre-validate each PDF so we can report which one(s) are broken
+                // Use the original MultipartFile to avoid deleting the tempFile during validation
+                try (PDDocument ignored = pdfDocumentFactory.load(multipartFile)) {
+                    // OK
+                } catch (IOException e) {
+                    ExceptionUtils.logException("PDF pre-validate", e);
+                    invalidIndexes.add(index);
+                }
                 mergerUtility.addSource(tempFile); // Add source file to the merger utility
             }
 
+            if (!invalidIndexes.isEmpty()) {
+                // Parse client file IDs (always present from frontend)
+                String[] clientIds = parseClientFileIds(request.getClientFileIds());
+                
+                // Map invalid indexes to client IDs
+                List<String> errorFileIds = new ArrayList<>();
+                for (Integer index : invalidIndexes) {
+                    if (index < clientIds.length) {
+                        errorFileIds.add(clientIds[index]);
+                    }
+                }
+                
+                String payload = String.format(
+                    "{\"errorFileIds\":%s,\"message\":\"Some of the selected files can't be merged\"}",
+                    errorFileIds.toString()
+                );
+                
+                return ResponseEntity.status(HttpStatus.UNPROCESSABLE_ENTITY)
+                        .header("Content-Type", MediaType.APPLICATION_JSON_VALUE)
+                        .body(payload.getBytes(StandardCharsets.UTF_8));
+            }
+
             mergedTempFile = Files.createTempFile("merged-", ".pdf").toFile();
             mergerUtility.setDestinationFileName(mergedTempFile.getAbsolutePath());
 

app/core/src/main/java/stirling/software/SPDF/model/api/general/MergePdfsRequest.java

@@ -39,4 +39,10 @@ public class MergePdfsRequest extends MultiplePDFFiles {
             requiredMode = Schema.RequiredMode.NOT_REQUIRED,
             defaultValue = "false")
     private boolean generateToc = false;
+
+    @Schema(
+            description =
+                    "JSON array of client-provided IDs for each uploaded file (same order as fileInput)",
+            requiredMode = Schema.RequiredMode.NOT_REQUIRED)
+    private String clientFileIds;
 }