Add swipe navigation to the photo detail page

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.

static/style.css

@@ -248,6 +248,33 @@ body {
   height: auto;
 }
 
+/* Swipe navigation. pan-y claims the horizontal axis for the gesture handler
+   while leaving vertical scrolling native. will-change is scoped to the two
+   transient states so the image is not permanently promoted to its own layer. */
+
+.dimg-wrap {
+  touch-action: pan-y;
+}
+
+.dimg-wrap.dragging {
+  user-select: none;
+  -webkit-user-select: none;
+}
+
+.dimg-wrap.dragging img {
+  transition: none;
+  will-change: transform;
+}
+
+.dimg-wrap.animating img {
+  transition: transform 180ms ease-out, opacity 180ms ease-out;
+  will-change: transform;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .dimg-wrap.animating img { transition: none; }
+}
+
 .dsidebar {
   width: 220px;
   flex-shrink: 0;