fix(backend/services): Preserve existing settings when updating user …

…settings

- Updated `updateSettingsById` function in `userService.js` to preserve existing settings when updating user settings.
- Ensured only provided settings fields are updated, leaving other fields unchanged.
- Validated and applied partial updates to the `settings` subdocument.

backend/src/services/userService.js

@@ -179,17 +179,22 @@ const getSettingsById = async (userId) => {
 /**
  * @function updateSettingsById - Update the settings of a user by ID.
  * @param {string} userId - The user's ID.
- * @param {Object} settings - The user's settings.
+ * @param {Object} settings - The user's settings to update.
  * @returns {Promise<Object>} - The updated user settings.
  * @throws {Error} - Throws an error if the user fails to update.
  */
 const updateSettingsById = async (userId, settings) => {
   if (!mongoose.Types.ObjectId.isValid(userId)) return null;
 
+  const updateFields = {};
+  for (const key in settings) {
+    updateFields[`settings.${key}`] = settings[key];
+  }
+
   try {
     const updatedUser = await User.findByIdAndUpdate(
       userId,
-      { settings },
+      { $set: updateFields },
       { new: true, projection: { settings: 1 } },
     );
     if (!updatedUser) throw new Error('User not found');