feat(feishu): support runtime domain override

[xukp20]

Summary

Add optional runtime domain override support for Feishu/Lark platform configuration.

Related to #402.

What changed

• allow projects.platforms.options.domain for feishu / lark
• use the custom domain for Feishu/Lark runtime SDK clients
• validate custom domain format during platform initialization
• keep default behavior unchanged when domain is not set
• document the new option in:
  • config.example.toml
  • docs/feishu.md
  • docs/usage.md
  • docs/usage.zh-CN.md
• add regression tests for:
  • custom domain override
  • invalid custom domain rejection

Scope

This PR only adds runtime platform domain override support.

It does not change the domain used by Feishu setup/onboarding commands such as:

• cc-connect feishu setup
• cc-connect feishu new
• cc-connect feishu bind

Those flows still use the existing built-in onboarding URLs.

Why

Current main already has internal runtime support for a Feishu/Lark domain concept:

• REST client can use lark.WithOpenBaseUrl(...)
• Feishu WebSocket client can use larkws.WithDomain(...)

But this capability was not exposed through config, so users could not actually use it.

This PR exposes that existing runtime capability in a minimal and backward-compatible way.

Validation

• gofmt -w platform/feishu/feishu.go platform/feishu/platform_test.go
• go test ./platform/feishu -v
• pnpm install --frozen-lockfile && pnpm build in web/
• go build ./...
• go test ./...

[chenhg5]

LGTM. Clean implementation of Feishu domain override.

Review summary:

• ✅ Backward compatible (optional config)
• ✅ URL validation included
• ✅ Documentation updated
• ✅ Tests added
• ✅ CI passes

Good feature for enterprise users with custom Feishu domains.