Shortcuts and direct links between settings and admin area

Summary

Add quick navigation to the settings and admin areas. Split out of #492.

Current behaviour

Reaching the admin page and the settings page takes several clicks through the user menu. The scan directory fix flow in particular bounces between admin (change path) and settings (trigger scan), which is what #492 was complaining about.

Proposed change

• Keyboard shortcuts, for example g then s for settings and g then a for admin, following the existing shortcut conventions in the frontend if any, and listed in the shortcuts help overlay.
• Direct links between the two pages where the workflow needs them, for example a "Scan now" button next to a user's scan directory in the admin user table, and a link to the admin user table from the settings scan section for superusers.