feat: support feishu (China) region alongside lark (international)

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Summary

All API endpoints were hardcoded to larksuite.com. Added LARK_REGION config to switch between Lark (international) and Feishu (China) environments.

Changes

• internal/config/config.go: Added region field, LARK_REGION env var (default lark), GetRegion() and GetBaseURL()
• internal/auth/oauth.go: Replaced hardcoded URL constants with dynamic functions based on region
• internal/api/client.go, internal/api/messages.go: baseURL replaced with config.GetBaseURL()

Usage

Set LARK_REGION=feishu (env var) or region: feishu (config.yaml) to use feishu.cn endpoints.

Region	Auth	API
lark (default)	accounts.larksuite.com	open.larksuite.com
feishu	accounts.feishu.cn	open.feishu.cn

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Reference

There is an existing PR in the upstream repo addressing the same region support:
yjwong#18

That PR also includes wiki command restructuring (doc wiki spaces, doc wiki list, doc wiki resolve), but it has merge conflicts and remains unmerged as of now.

Our implementation focuses on the region-switching part only, using a similar approach (config-driven region field with lark/feishu values).