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/api/settings.ts

@@ -9,11 +9,11 @@ export interface UserConfig {
 }
 
 export interface UserConfigUpdate {
-  default_editor?: string;
-  theme?: string;
-  git_user_name?: string;
-  git_user_email?: string;
-  last_session_id?: string;
+  default_editor?: string | null;
+  theme?: string | null;
+  git_user_name?: string | null;
+  git_user_email?: string | null;
+  last_session_id?: string | null;
 }
 
 export const getUserConfig = async (): Promise<UserConfig> => {