feat(providers): add Alibaba (China) provider for the dashscope China endpoint

Summary

Maka's alibaba provider is pinned to the international Model Studio endpoint (https://dashscope-intl.aliyuncs.com/compatible-mode/v1). There is no provider for the China platform endpoint (https://dashscope.aliyuncs.com/compatible-mode/v1), so China-platform users cannot connect with their sk-xxx keys issued on bailian.console.aliyun.com — and cannot select China-platform models such as qwen3.8-max.

Other tools consuming models.dev already expose this as Alibaba (China) (alibaba-cn), e.g. OpenCode offers qwen3.8-max through it.

Evidence (checked 2026-08-20)

• models.dev publishes alibaba-cn ("Alibaba (China)", https://dashscope.aliyuncs.com/compatible-mode/v1) with a full tool-calling catalog including qwen3.8-max (1M context, 128k output, vision/pdf input, thinking toggle + efforts).
• Maka's registry (packages/core/src/provider-registry.ts) binds alibaba to the intl endpoint only; scripts/sync-model-metadata.mjs does not ingest alibaba-cn, so no China access path exists.
• Distinct from #3314: that tracks qwen3.8-max on the Coding Plan subscription (blocked on Alibaba's plan allowlist); this issue is the pay-as-you-go China endpoint, which already serves qwen3.8-max today.

Proposed change (mirrors the existing alibaba shape)

1. scripts/sync-model-metadata.mjs: ingest alibaba-cn; regenerate model-metadata.generated.ts / model-pricing.generated.ts (pay-as-you-go, so priced like alibaba).
2. packages/core/src/provider-registry.ts: validate models.dev alibaba-cn facts, derive tool-calling model ids, register alibaba-cn (domestic, API group, signup https://bailian.console.aliyun.com/).
3. Desktop settings: display copy (Alibaba 中国站 / Alibaba (China)) and brand mark.