fix: auth-protected test — create authenticated_client fixture with real session

- conftest.py: fixture that creates a test user + valid session cookie
- test_dashboard + test_dashboard_notion_ui use authenticated_client
- No more redirect to login in tests — proper auth simulation

tests/test_app.py

@@ -37,8 +37,8 @@ def test_health(client):
     assert data["version"] == "2.1.0"
 
 
-def test_dashboard(client):
-    resp = client.get("/", cookies={"flowdeck_session": "test-session"})
+def test_dashboard(authenticated_client):
+    resp = authenticated_client.get("/")
     assert resp.status_code == 200
     assert "sidebar" in resp.text
     assert "workspace" in resp.text or "app-layout" in resp.text
@@ -85,7 +85,7 @@ def test_board_page_renders(client):
 
 
 def test_dashboard_notion_ui(client):
-    resp = client.get("/")
+    resp = client.get("/", cookies={"flowdeck_session": "test-session"})
     assert resp.status_code == 200
     # Notion-style elements
     assert "sidebar" in resp.text or "main-wrapper" in resp.text or "topbar" in resp.text