Set default encryption key length to 128 bit AES in add-password.html (#3693)

# Description of Changes

This pull request makes a minor update to the `add-password.html`
template to improve the user experience by setting the default selection
for the key length to `128`.

*
[`stirling-pdf/src/main/resources/templates/security/add-password.html`](diffhunk://#diff-63c5e71a044226ec14b6c30b73cd94ab0b3c9b0606dc93ef4c942f2587a64822L36-R36):
Updated the `<option>` element for the key length `128` to include the
`selected` attribute, making it the default choice in the dropdown menu.


![image](https://github.com/user-attachments/assets/49bbf64a-7409-4259-837d-07b8d8ab280a)


Closes #3516

---

## 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)
- [ ] 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

- [ ] 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/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)

### 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.

stirling-pdf/src/main/resources/templates/security/add-password.html

@@ -33,7 +33,7 @@
                 <div class="mb-3">
                   <label th:text="#{addPassword.selectText.3}"></label> <select class="form-control" id="keyLength" name="keyLength">
                     <option value="40">40</option>
-                    <option value="128">128</option>
+                    <option value="128" selected>128</option>
                     <option value="256">256</option>
                   </select>
                   <small class="form-text text-muted" th:text="#{addPassword.selectText.4}"></small>