feat: Add comprehensive Chinese localization support for issue #412 (#649)

* 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

src/server/app.py

@@ -126,6 +126,7 @@ async def chat_stream(request: ChatRequest):
             request.enable_deep_thinking,
             request.enable_clarification,
             request.max_clarification_rounds,
+            request.locale,
         ),
         media_type="text/event-stream",
     )
@@ -307,6 +308,7 @@ async def _astream_workflow_generator(
     enable_deep_thinking: bool,
     enable_clarification: bool,
     max_clarification_rounds: int,
+    locale: str = "en-US",
 ):
     # Process initial messages
     for message in messages:
@@ -335,6 +337,7 @@ async def _astream_workflow_generator(
         "clarified_research_topic": clarified_research_topic,
         "enable_clarification": enable_clarification,
         "max_clarification_rounds": max_clarification_rounds,
+        "locale": locale,
     }
 
     if not auto_accepted_plan and interrupt_feedback: