Swipe left for older, right for newer, matching the existing arrow keys.
The image tracks the finger and slides out on commit; the exit animation
covers the page load, so this stays plain static-site navigation.
The keyboard handler moves out of the template into static/photo-nav.js,
which the existing //go:embed of static/ picks up with no generator
changes.
Swipes starting within 24px of a screen edge are left alone — that strip
is the browser's own back-gesture. Only touch pointers are handled, so
desktop drag-to-save still works, and the axis locks after 10px so
vertical drags stay scrolls. With no neighbour that way the image
rubber-bands instead of navigating.
Also prefetch the neighbour medium JPEGs, not just their HTML, or the
next page still blocks on a fresh image request.
The keyboard handler now bails on modifier keys; it previously hijacked
Alt+ArrowLeft, which is browser-back on Linux and Windows.
