feat: build Streamlit chat interface for support copilot

ui/app.py

@@ -0,0 +1,43 @@
+import streamlit as st
+import requests
+import os
+
+st.set_page_config(page_title="Support Docs Copilot", page_icon="🤖")
+st.title("🤖 Support Docs Copilot")
+
+BACKEND_URL = os.getenv("BACKEND_URL", "http://127.0.0.1:8000/chat/stream")
+
+if "messages" not in st.session_state:
+    st.session_state.messages = []
+
+for message in st.session_state.messages:
+    with st.chat_message(message["role"]):
+        st.markdown(message["content"])
+
+if user_query := st.chat_input("Ask a support question..."):
+    with st.chat_message("user"):
+        st.markdown(user_query)
+    st.session_state.messages.append({"role": "user", "content": user_query})
+
+    with st.chat_message("assistant"):
+        response_placeholder = st.empty()
+        full_response = ""
+        try:
+            with requests.post(BACKEND_URL, json={"query": user_query}, stream=True) as response:
+                if response.status_code == 200:
+                    for chunk in response.iter_content(chunk_size=None, decode_unicode=True):
+                        if chunk:
+                            full_response += chunk
+                            response_placeholder.markdown(full_response + "▌")
+                    response_placeholder.markdown(full_response)
+                elif response.status_code == 400:
+                    full_response = f"⚠️ {response.json().get('detail', 'Violation.')}"
+                    response_placeholder.error(full_response)
+                else:
+                    full_response = "⚠️ Server communication error."
+                    response_placeholder.error(full_response)
+        except:
+            full_response = "❌ Backend connection error."
+            response_placeholder.error(full_response)
+
+    st.session_state.messages.append({"role": "assistant", "content": full_response})