feat: add JupyterHub dynamic prefix support

[liuji1031]

Summary:
Enable Superset to run under JupyterHub's dynamic per-user URL prefixes (e.g., /user/username/).

Backend:

• Inject JUPYTERHUB_SERVICE_PREFIX into bootstrap data for frontend consumption
• Prefix user menu URLs (Info, Logout) that bypass Flask's url_for()
• Handle prefixed redirects in login flow with proper next parameter support

Frontend:

• Configure SupersetClient with application_root from bootstrap data
• Use SupersetClient.getUrl() for dataset creation navigation links

Deployment:

• Works with jhsingle-native-proxy (strips incoming prefix, preserves in responses via middleware)
• Compatible with Flask dev server for single-user container deployments