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.

src/main/java/stirling/software/SPDF/controller/web/AccountWebController.java

@@ -191,7 +191,7 @@ public class AccountWebController {
         }
 
         if (request.getParameter("logout") != null) {
-            model.addAttribute("logoutMessage", "You have been logged out.");
+            model.addAttribute("logoutMessage", "login.logoutMessage");
         }
 
         return "login";

src/main/resources/messages_de_DE.properties

@@ -607,6 +607,7 @@ login.userIsDisabled=Benutzer ist deaktiviert, die Anmeldung ist mit diesem Benu
 login.alreadyLoggedIn=Sie sind bereits an
 login.alreadyLoggedIn2=Geräten angemeldet. Bitte melden Sie sich dort ab und versuchen es dann erneut.
 login.toManySessions=Sie haben zu viele aktive Sitzungen
+login.logoutMessage=Sie wurden erfolgreich abgemeldet.
 
 #auto-redact
 autoRedact.title=Automatisch zensieren/schwärzen

src/main/resources/messages_en_GB.properties

@@ -609,6 +609,7 @@ login.userIsDisabled=User is deactivated, login is currently blocked with this u
 login.alreadyLoggedIn=You are already logged in to
 login.alreadyLoggedIn2=devices. Please log out of the devices and try again.
 login.toManySessions=You have too many active sessions
+login.logoutMessage=You have been logged out.
 
 #auto-redact
 autoRedact.title=Auto Redact
@@ -1432,7 +1433,7 @@ cookieBanner.preferencesModal.description.2=Stirling PDF cannot—and will never
 cookieBanner.preferencesModal.description.3=Your privacy and trust are at the core of what we do.
 cookieBanner.preferencesModal.necessary.title.1=Strictly Necessary Cookies
 cookieBanner.preferencesModal.necessary.title.2=Always Enabled
-cookieBanner.preferencesModal.necessary.description=These cookies are essential for the website to function properly. They enable core features like setting your privacy preferences, logging in, and filling out forms—which is why they can’t be turned off. 
+cookieBanner.preferencesModal.necessary.description=These cookies are essential for the website to function properly. They enable core features like setting your privacy preferences, logging in, and filling out forms—which is why they can’t be turned off.
 cookieBanner.preferencesModal.analytics.title=Analytics
 cookieBanner.preferencesModal.analytics.description=These cookies help us understand how our tools are being used, so we can focus on building the features our community values most. Rest assured—Stirling PDF cannot and will never track the content of the documents you work with.
 

src/main/resources/templates/login.html

@@ -112,7 +112,7 @@
             <div th:if="${error}" th:text="#{${error}}">OAuth2: Error Message</div>
           </div>
           <div class="text-danger text-center">
-            <div th:if="${logoutMessage}" class="alert alert-success" th:text="${logoutMessage}"></div>
+            <div th:if="${logoutMessage}" class="alert alert-success" th:text="#{${logoutMessage}}"></div>
             <div th:if="${messageType}" class="alert alert-success">
               <span th:text="#{${messageType}}">Default message if not found</span>
             </div>