- Add login and logout icon macros to shared icons partial
- Desktop nav uses icon-only with aria-label for both
- Mobile nav uses icon + text for both
- Add checkin map-pin icon to shared icons partial
- Add check-in nav link (icon on desktop, icon + text on mobile)
- Only shown when authenticated
- Set nav_active to "checkin" on the check-in page
Replace duplicated inline SVGs across 16 templates with reusable Askama
macros from a new icons.html partial, reducing repetition and making
icon updates a single-point change.
