feat(ui): explicit chat state machine — typing indicator, streaming progress, timeout and error recovery

tests/integration/test_chat_api.py

@@ -264,6 +264,24 @@ def test_chat_mid_stream_failure_yields_error_after_partial_deltas(client, db) -
     assert db.scalars(select(QueryLog)).all() == []
 
 
+def test_error_event_matches_contract_shape(client, db, seeded_kb) -> None:
+    """The SSE error event (PLAN §4) is exactly ``{type, detail}`` — the
+    client's loading-feedback state machine (phase 06) keys off this shape
+    to flip to the error state and re-enable the send button."""
+    broken = FakeRagLLM(embed_error=EmbeddingError("embeddings endpoint down"))
+    fastapi_app.dependency_overrides[chat_api.get_llm] = lambda: broken
+    try:
+        _, _, frames = _stream_chat(client, QUESTION)
+    finally:
+        fastapi_app.dependency_overrides.clear()
+
+    assert len(frames) == 1
+    event = frames[0]
+    assert set(event.keys()) == {"type", "detail"}
+    assert event["type"] == "error"
+    assert isinstance(event["detail"], str) and event["detail"]
+
+
 def test_chat_db_down_returns_503_json(client, monkeypatch) -> None:
     monkeypatch.setattr(chat_api, "db_available", lambda: False)
     r = client.post("/api/chat", json={"message": "hello"})