Validation for input file (#3196)

# Description of Changes

Please provide a summary of the changes, including:

- Added an invalid event listener for the input element, which will be
triggered if the input is empty and the element is required in the form.
This is a global change, where all pages using this component will have
this functionality.

Closes #1841

---

## Checklist

### General

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

### Documentation

- [x] 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)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

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

![image](https://github.com/user-attachments/assets/c53af9fa-b933-420f-9ee3-7b4600c44ea4)

![image](https://github.com/user-attachments/assets/8da04fa1-58be-42eb-977a-c850f917c011)

![image](https://github.com/user-attachments/assets/48dcce19-68c3-4676-a6d2-28b1b0e86004)

### Testing (if applicable)

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

---------

Co-authored-by: Anthony Stirling <[email protected]>

src/main/resources/messages_en_GB.properties

@@ -83,6 +83,7 @@ loading=Loading...
 addToDoc=Add to Document
 reset=Reset
 apply=Apply
+noFileSelected=No file selected. Please upload one.
 
 legal.privacy=Privacy Policy
 legal.terms=Terms and Conditions
@@ -1308,7 +1309,7 @@ survey.button=Take Survey
 survey.dontShowAgain=Don't show again
 survey.meeting.1=If you're using Stirling PDF at work, we'd love to speak to you. We're offering technical support sessions in exchange for a 15 minute user discovery session.
 survey.meeting.2=This is a chance to:
-survey.meeting.3=Get help with deployment, integrations, or troubleshooting 
+survey.meeting.3=Get help with deployment, integrations, or troubleshooting
 survey.meeting.4=Provide direct feedback on performance, edge cases, and feature gaps
 survey.meeting.5=Help us refine Stirling PDF for real-world enterprise use
 survey.meeting.6=If you're interested, you can book time with our team directly. (English speaking only)