feat: Add light/dark mode support with backend persistence

backend/tests/api/test_theme.py

@@ -0,0 +1,73 @@
+import pytest
+from fastapi.testclient import TestClient
+from unittest.mock import patch
+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 test_user():
+    return User(
+        id="user-123",
+        username="test@example.com",
+        display_name="Test User",
+        theme_preference="light"
+    )
+
+@pytest.fixture
+def auth_token():
+    return create_access_token(data={"sub": "user-123"})
+
+def test_get_me_includes_theme(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
+        
+        response = client.get(
+            "/api/v1/auth/me",
+            cookies={"access_token": auth_token}
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert "theme_preference" in data
+        assert data["theme_preference"] == "light"
+
+def test_update_theme_success(test_user, auth_token):
+    """Test successful theme update via PATCH /auth/theme."""
+    updated_user = User(
+        id="user-123",
+        username="test@example.com",
+        display_name="Test User",
+        theme_preference="dark"
+    )
+    
+    with patch("ea_chatbot.api.dependencies.history_manager") as mock_hm_dep, \
+         patch("ea_chatbot.api.routers.auth.history_manager") as mock_hm_router:
+        
+        # Dependency injection uses the one from dependencies
+        mock_hm_dep.get_user_by_id.return_value = test_user
+        
+        # The router uses its own reference to history_manager
+        mock_hm_router.update_user_theme.return_value = updated_user
+        
+        response = client.patch(
+            "/api/v1/auth/theme",
+            json={"theme": "dark"},
+            cookies={"access_token": auth_token}
+        )
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["theme_preference"] == "dark"
+        mock_hm_router.update_user_theme.assert_called_once_with("user-123", "dark")
+
+def test_update_theme_unauthorized():
+    """Test that theme update requires authentication."""
+    response = client.patch(
+        "/api/v1/auth/theme",
+        json={"theme": "dark"}
+    )
+    assert response.status_code == 401