Make model ids configurable via environment variables

[amm10090]

Summary

• add environment-variable overrides for both image and svg model IDs and keep defaults on the free-tier models so npx works without paid credits
• update configuration docs with the new defaults and step-by-step examples for switching to paid models (Flux Schnell + Recraft V3 SVG) or any other Replicate model

Testing

• not run (docs + config defaults only)

[amm10090]

Update: Added Chinese README (README.zh.md) plus language toggle links at the top of both READMEs. The Chinese doc mirrors quick start, integrations (including Codex), model config, and troubleshooting, and links to Replicate’s Try for Free collection for choosing free models.