docs: sync model pricing and promotions

[ai-bankofai]

Summary

• Align English and Chinese LLM model, pricing, and promotion docs with the canonical bai-documents source.
• Update GLM-5.3 cache-read pricing from $0.26 to $0.28 per 1M tokens and the 10%-off promotional price from $0.234 to $0.252.
• Add the reusable ActivityCard component for free, discount, and price-adjustment notices.
• Bump the documentation version from 1.3.20 to 1.3.21.

Validation

• yarn build (English and Chinese builds completed successfully; existing unrelated broken-link warnings remain).