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

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

backend/app/routers/generate.py

@@ -23,10 +23,17 @@ async def generate_text(
     _: dict = Depends(get_current_user),
 ) -> TextResponse:
     """Generate text from a prompt using a chat model."""
-    messages = []
-    if body.system_prompt:
-        messages.append({"role": "system", "content": body.system_prompt})
-    messages.append({"role": "user", "content": body.prompt})
+    if body.messages:
+        messages = [{"role": m.role, "content": m.content}
+                    for m in body.messages]
+        if body.system_prompt and (not messages or messages[0]["role"] != "system"):
+            messages.insert(
+                0, {"role": "system", "content": body.system_prompt})
+    else:
+        messages = []
+        if body.system_prompt:
+            messages.append({"role": "system", "content": body.system_prompt})
+        messages.append({"role": "user", "content": body.prompt})
 
     try:
         result = await openrouter.chat_completion(