fix(settings): properly save and clear configuration values

Backend:
- Replace dict mutation with dict replacement to fix SQLAlchemy JSON
  mutation tracking issue (config.config = {**config.config, **update_data})

Frontend:
- Send explicit null values instead of undefined so fields can be cleared
- Update UserConfigUpdate interface to accept null values

apps/web/src/pages/settings.tsx

@@ -46,9 +46,9 @@ export const SettingsPage = () => {
     try {
       const update: UserConfigUpdate = {
         theme: config.theme,
-        default_editor: config.default_editor ?? undefined,
-        git_user_name: config.git_user_name ?? undefined,
-        git_user_email: config.git_user_email ?? undefined,
+        default_editor: config.default_editor,
+        git_user_name: config.git_user_name,
+        git_user_email: config.git_user_email,
       };
       console.log("Sending update:", update);
       const updated = await updateUserConfig(update);