Add Setting Env

This walks you through wiring up environment variables that set default values for user settings in a Next.js app using t3-oss/env-nextjs and Zod validation. You define the env var with schema validation, optionally create new domain types in your server config, merge them into the user store, and document everything in both English and Chinese. The priority hierarchy is clear: user customs override server env vars, which override hardcoded defaults. It's thorough enough that you won't forget the .env.example update or the docs, which honestly happens more often than it should. The AI_IMAGE_DEFAULT_IMAGE_NUM example shows the full flow from validation (1-20 range) through to the store slice.