feat: support custom LLM headers

[RuoJi6]

Summary

• Add optional custom HTTP headers for LLM provider configuration.
• Support headers from YAML, repeated --llm-header KEY=VALUE, and the Web UI/API.
• Apply custom headers last for OpenAI-compatible and Anthropic chat/web-search requests so explicit User-Agent values can override defaults.
• Validate and canonicalize header names, reject CR/LF values, and ignore empty header rows.

Validation

• git diff --check upstream/master...HEAD
• go test ./core/config ./pkg/agent/provider ./pkg/web
• npm run build in web/frontend

Notes

• Empty or omitted headers preserves existing default request header behavior.
• This is intended for OpenAI-compatible providers that require headers such as User-Agent: Version: 5.10.0 openwarp.