* feat: Add comprehensive Chinese localization support for issue #412
- Add locale parameter to ChatRequest model to capture user's language preference
- Implement language-aware template loading in template.py with fallback to English
- Update all apply_prompt_template calls to pass locale through the workflow
- Create Chinese translations for 14 core prompt files:
* Main agents: coordinator, planner, researcher, reporter, coder
* Subprocess agents: podcast_script_writer, ppt_composer, prompt_enhancer
* Writing assistant: all 6 prose prompts
- Update app.py to extract and propagate locale through workflow state
- Support both zh-CN and en-US locales with automatic fallback
- Ensure locale flows through all agent nodes and template rendering
* address the review suggestions
