Add newer/older navigation to the photo detail page

Replace the standalone Back link with a three-cell nav row so each
photo page links to its neighbours in the timeline. Prefetch the
neighbouring pages and bind the arrow keys for keyboard navigation.

static/style.css

@@ -195,17 +195,34 @@ body {
   padding: 16px 20px;
 }
 
-.back-btn {
-  display: inline-block;
-  font-size: 15px;
-  color: var(--fg3);
-  text-decoration: none;
+.photo-nav {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
   padding: 8px 0;
   margin-bottom: 16px;
-  font-family: var(--serif);
+  font-family: monospace;
+  font-size: 13px;
+  letter-spacing: 0.04em;
 }
 
-.back-btn:hover {
+.photo-nav a {
+  color: var(--aqua);
+  text-decoration: none;
+}
+
+.photo-nav a:hover {
+  color: var(--fg2);
+}
+
+.photo-nav .back-btn {
+  font-family: var(--serif);
+  font-size: 15px;
+  color: var(--fg3);
+  letter-spacing: 0;
+}
+
+.photo-nav .back-btn:hover {
   color: var(--fg);
 }