fix(app): handle missing ChromaDB collection gracefully on startup

Space crashes if pipeline data hasn't been uploaded. Wrap collection
load in try/except; show a clear message in UI instead of crashing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -37,12 +37,19 @@ def _load_trials() -> list[dict]:
         return [json.loads(line) for line in f if line.strip()]
 
 
-_collection = load_collection(CHROMA_DIR, CHROMA_COLLECTION)
+def _load_collection():
+    try:
+        return load_collection(CHROMA_DIR, CHROMA_COLLECTION)
+    except Exception:
+        _logger.warning("ChromaDB collection not found — running in demo mode (no data)")
+        return None
+
+_collection = _load_collection()
 _graph = _load_graph()
 _trials = _load_trials()
 _client = anthropic.Anthropic()
 
-_n_chunks = _collection.count()
+_n_chunks = _collection.count() if _collection else 0
 _n_trials = len(_trials)
 _kg_nodes = _graph.number_of_nodes() if _graph else 0
 
@@ -64,6 +71,12 @@ def respond(message: str, history: list[dict]):
         yield history, gr.update(value="", interactive=True)
         return
 
+    if _collection is None:
+        history = history + [{"role": "user", "content": message}]
+        history = history + [{"role": "assistant", "content": "⚠️ The knowledge base has not been loaded yet. The pipeline data (ChromaDB index, knowledge graph, papers) needs to be uploaded to this Space. Please contact the Space administrator."}]
+        yield history, gr.update(value="", interactive=True)
+        return
+
     history = history + [{"role": "user", "content": message}]
     history = history + [{"role": "assistant", "content": ""}]
     yield history, gr.update(value="", interactive=False)