Fix empty-parameter issue in updateUserSettings by using @RequestBody map (#3536)

# Description of Changes

Please provide a summary of the changes, including:


- **What was changed:**  
- Refactored the `updateUserSettings` method in `UserController` to
accept a `@RequestBody Map<String, String>` named `updates` instead of
pulling parameters from `HttpServletRequest`.
- Removed the now-unused `HashMap` import and the manual
parameter-extraction loop.

- **Why the change was made:**  
- **Bug Fix:** The previous implementation relied on
`request.getParameterMap()`, which was consistently empty, so no
settings were ever applied.
- Simplifies controller logic by leveraging Spring’s request-body
binding.
- Improves readability and maintainability, removing boilerplate and
error-prone code.

---

## 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)

- [ ] 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.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/main/java/stirling/software/SPDF/controller/api/UserController.java

@@ -3,7 +3,6 @@ package stirling.software.SPDF.controller.api;
 import java.io.IOException;
 import java.security.Principal;
 import java.sql.SQLException;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -168,13 +167,23 @@ public class UserController {
 
     @PreAuthorize("!hasAuthority('ROLE_DEMO_USER')")
     @PostMapping("/updateUserSettings")
-    public String updateUserSettings(HttpServletRequest request, Principal principal)
+    /**
+     * Updates the user settings based on the provided JSON payload.
+     *
+     * @param updates A map containing the settings to update. The expected structure is:
+     *                <ul>
+     *                  <li><b>emailNotifications</b> (optional): "true" or "false" - Enable or disable email notifications.</li>
+     *                  <li><b>theme</b> (optional): "light" or "dark" - Set the user's preferred theme.</li>
+     *                  <li><b>language</b> (optional): A string representing the preferred language (e.g., "en", "fr").</li>
+     *                </ul>
+     *                Keys not listed above will be ignored.
+     * @param principal The currently authenticated user.
+     * @return A redirect string to the account page after updating the settings.
+     * @throws SQLException If a database error occurs.
+     * @throws UnsupportedProviderException If the operation is not supported for the user's provider.
+     */
+    public String updateUserSettings(@RequestBody Map<String, String> updates, Principal principal)
             throws SQLException, UnsupportedProviderException {
-        Map<String, String[]> paramMap = request.getParameterMap();
-        Map<String, String> updates = new HashMap<>();
-        for (Map.Entry<String, String[]> entry : paramMap.entrySet()) {
-            updates.put(entry.getKey(), entry.getValue()[0]);
-        }
         log.debug("Processed updates: {}", updates);
         // Assuming you have a method in userService to update the settings for a user
         userService.updateUserSettings(principal.getName(), updates);