style: 简化文档卡片样式，优化布局间距

backend/llm.py

@@ -63,10 +63,10 @@ def _extract_message(response) -> tuple[str, str]:
 async def call_ollama(
     prompt: str,
     *,
-    system_prompt: str = None,
+    system_prompt: str | None = None,
     tag: str = "default",
     temperature: float = 0.7,
-    thinking: str = None,
+    thinking: str | None = None,
 ) -> dict:
     """
     调用 Ollama API 并返回 content 和 thinking。