docs: sync API reference and latest model pricing

[ai-bankofai]

Summary

• Sync the English and Chinese API Reference, including DeepSeek Responses support and Codex web-search configuration.
• Add English and Chinese pages for Claude Fable 5.1, Gemini 3.8 Flash, Muse Spark 1.3, Hy4 Preview, and DeepSeek-V4-Flash-Vision-Exp.
• Update model pricing, DeepSeek Idle/Busy periods, and the 50% API discount effective September 3, 2026.
• Update sidebar navigation and bump the documentation version from 1.3.26 to 1.3.27.

Validation

• yarn build passes for English and Chinese (existing unrelated broken-link warnings only).