Implement chat interface with message history and system prompt support; update frontend and tests accordingly

Co-authored-by: Copilot <copilot@github.com>

backend/app/models/ai.py

@@ -33,8 +33,9 @@ class ModelInfo(BaseModel):
 
 class TextRequest(BaseModel):
     model: str
-    prompt: str
+    prompt: str = ""
     system_prompt: str | None = None
+    messages: list[ChatMessage] | None = None
     temperature: float = 0.7
     max_tokens: int = 1024