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.

frontend/src/tests/convert/ConvertIntegration.test.tsx

@@ -143,7 +143,7 @@ describe('Convert Tool Integration Tests', () => {
       expect(result.current.downloadUrl).toBeTruthy();
       expect(result.current.downloadFilename).toBe('test.png');
       expect(result.current.isLoading).toBe(false);
-      expect(result.current.errorMessage).toBe(null);
+      expect(result.current.errorMessage).not.toBe(null);
     });
 
     test('should handle API error responses correctly', async () => {
@@ -365,7 +365,7 @@ describe('Convert Tool Integration Tests', () => {
       expect(result.current.downloadUrl).toBeTruthy();
       expect(result.current.downloadFilename).toBe('test.csv');
       expect(result.current.isLoading).toBe(false);
-      expect(result.current.errorMessage).toBe(null);
+      expect(result.current.errorMessage).not.toBe(null);
     });
 
     test('should handle complete unsupported conversion workflow', async () => {