Internationalize logout message (#3450)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
- Controller now uses the `login.logoutMessage` i18n key instead of
hard-coded text.
- Added `login.logoutMessage` entry to `messages_de_DE.properties` and
`messages_en_GB.properties`.
- Updated `login.html` to resolve the logout message via
`th:text="#{…}"`.

- **Why the change was made**  
  - To support localization for logout feedback.
- To eliminate hard-coded strings from the view layer and rely on
message bundles.

before:


![image](https://github.com/user-attachments/assets/5e9975f6-717f-4035-8e3c-76df8c0275bb)


after:


![image](https://github.com/user-attachments/assets/934f45ad-d490-4a34-9399-5c9031f2db2d)


---

## 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)

- [ ] 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.