feat: add Gradio web UI

- app.py: Gradio Interface wrapping BoschProductRAG.query()
- Index built once at startup (not per-request)
- Markdown output: answer + linked sources + latency
- Example prompts included for quick demo
- Verified: launches on :7860, returns correct answers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app.py

@@ -0,0 +1,60 @@
+"""Gradio web UI for the Bosch HVAC Product Knowledge Bot.
+
+Run with: python app.py
+"""
+import os
+from dotenv import load_dotenv
+import gradio as gr
+
+from src.rag import BoschProductRAG
+
+load_dotenv()
+
+CATALOG_PATH = os.path.join(os.path.dirname(__file__), "src", "products_catalog.json")
+
+# Build the RAG index once at startup (not per-request).
+_rag = BoschProductRAG()
+_rag.build_index(_rag.load_products(CATALOG_PATH))
+
+
+def answer_question(question: str) -> str:
+    if not question or not question.strip():
+        return "Please enter a question."
+
+    result = _rag.query(question)
+
+    sources_md = "\n".join(
+        f"- [{s['name']}]({s['url']})" if s["url"] else f"- {s['name']}"
+        for s in result["sources"]
+    )
+
+    return (
+        f"### Answer\n{result['answer']}\n\n"
+        f"### Sources\n{sources_md or '_none retrieved_'}\n\n"
+        f"---\n*Latency: {result['latency_ms']:.0f}ms*"
+    )
+
+
+demo = gr.Interface(
+    fn=answer_question,
+    inputs=gr.Textbox(
+        label="Ask about Bosch HVAC products",
+        placeholder="e.g. Which heat pump is best for retrofits?",
+        lines=2,
+    ),
+    outputs=gr.Markdown(label="Response"),
+    title="Bosch HVAC Product Knowledge Bot",
+    description=(
+        "RAG-powered assistant for Bosch HVAC sales & spec questions. "
+        "Retrieval: FAISS + Google Gemini embeddings. Generation: Gemini Flash Lite."
+    ),
+    examples=[
+        "Which heat pump is best for retrofits?",
+        "What's the most energy-efficient system you offer?",
+        "Do you have a smart thermostat?",
+        "What cooling systems use R32 refrigerant?",
+    ],
+)
+
+if __name__ == "__main__":
+    demo.launch()