fix: Address second code review findings (test isolation, frontend type safety)

backend/tests/api/test_theme.py

@@ -5,7 +5,9 @@ from ea_chatbot.api.main import app
 from ea_chatbot.history.models import User
 from ea_chatbot.api.utils import create_access_token
 
-client = TestClient(app)
+@pytest.fixture
+def client():
+    return TestClient(app)
 
 @pytest.fixture
 def test_user():
@@ -20,7 +22,7 @@ def test_user():
 def auth_token():
     return create_access_token(data={"sub": "user-123"})
 
-def test_get_me_includes_theme(test_user, auth_token):
+def test_get_me_includes_theme(client, test_user, auth_token):
     """Test that /auth/me returns the theme_preference."""
     with patch("ea_chatbot.api.dependencies.history_manager") as mock_hm:
         mock_hm.get_user_by_id.return_value = test_user
@@ -33,7 +35,7 @@ def test_get_me_includes_theme(test_user, auth_token):
         assert "theme_preference" in data
         assert data["theme_preference"] == "light"
 
-def test_update_theme_success(test_user, auth_token):
+def test_update_theme_success(client, test_user, auth_token):
     """Test successful theme update via PATCH /auth/theme."""
     updated_user = User(
         id="user-123",
@@ -62,7 +64,7 @@ def test_update_theme_success(test_user, auth_token):
         assert data["theme_preference"] == "dark"
         mock_hm_router.update_user_theme.assert_called_once_with("user-123", "dark")
 
-def test_update_theme_unauthorized():
+def test_update_theme_unauthorized(client):
     """Test that theme update requires authentication."""
     response = client.patch(
         "/api/v1/auth/theme",