feat(admin-ui): support multi-repository SNAuth sessions

[VargaJoe]

Summary

• Scope SNAuth config and tokens per normalized repository URL.
• Add recent SNAuth repositories on login and an authenticated repository selector in the Admin UI drawer.
• Add repository/session cleanup paths and an app error fallback for repository loading failures.

Verification

• git diff origin/feature/sn-auth-package-extraimprovements...HEAD --check
• npx --yes yarn@1.22.22 workspace @sensenet/sn-auth-react build:types
• npx --yes yarn@1.22.22 workspace @app/sensenet build:snauth
• npx --yes yarn@1.22.22 workspace @app/sensenet build:idserver

Builds completed with the existing webpack asset-size and outdated Browserslist warnings.

Review note

Please pay special attention to the automatic IdentityServer -> SNAuth fallback path. The current flow sets the SNAuth pending-login marker in changeAuthType(), then RepositoryProvider clears active repository selection on the incompatible auth-server branch; that can clear the pending marker before SnAuthRepositoryProvider decides whether to call externalLogin().