v1.6.1 - Admin audit and responsive navigation

v1.6.1 — Admin audit and responsive navigation

Added

• Admin audit events for user create/delete, role changes, block/unblock and password reset.
• Audit events for temporary user creation.
• Audit events for local and Let's Encrypt private key downloads.
• Audit events for backup downloads.
• Audit events for notification settings and Let's Encrypt settings changes without logging secrets.
• Responsive top navigation with dropdown groups for narrow windows in both the main certificate UI and admin workspace.

Changed

• Security log now distinguishes login, logout, 2FA, audit and failed events.
• README badges and current release blocks updated to v1.6.1.

Verified

• go test ./...
• docker compose build step-ui
• local step-ui container healthy after rebuild