fix(cookbook): locate cookbook_state.json via DATA_DIR, not hardcoded /app/data (#3332)

Three call sites hardcoded Path("/app/data/cookbook_state.json"), which only
exists in Docker; on a native run the real path is <repo>/data, so the state
file looked missing and cookbook serve-state was silently ignored. Two others
used os.environ.get("DATA_DIR", "data") (a relative fallback, since DATA_DIR is
never set as an env var). Route all five through core.constants.DATA_DIR so the
path is consistent and absolute on both Docker and native.

Part of #3331.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

routes/cookbook_routes.py

@@ -33,6 +33,7 @@ from core.platform_compat import (
     get_wsl_windows_user_profile,
 )
 from routes.shell_routes import TMUX_LOG_DIR
+from core.constants import DATA_DIR
 
 logger = logging.getLogger(__name__)
 
@@ -60,7 +61,7 @@ _HF_TOKEN_STATUS_SNIPPET = (
 
 def setup_cookbook_routes() -> APIRouter:
     router = APIRouter(tags=["cookbook"])
-    _cookbook_state_path = Path(os.environ.get("DATA_DIR", "data")) / "cookbook_state.json"
+    _cookbook_state_path = Path(DATA_DIR) / "cookbook_state.json"
 
     def _mask_secret(value: str) -> str:
         if not value: