Split the photo nav so Back sits alone on the left

Move the Newer and Older links to a right-aligned group joined by a
slash so they stop crowding the Back link, and drop the aqua accent so
every link in the row shares the muted serif tone.

static/style.css

@@ -198,34 +198,26 @@ body {
 .photo-nav {
   display: flex;
   justify-content: space-between;
-  align-items: center;
+  align-items: baseline;
   padding: 8px 0;
   margin-bottom: 16px;
-  font-family: monospace;
-  font-size: 13px;
-  letter-spacing: 0.04em;
+  font-family: var(--serif);
+  font-size: 15px;
 }
 
 .photo-nav a {
-  color: var(--aqua);
+  color: var(--fg3);
   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);
 }
 
+.photo-nav-neighbours {
+  color: var(--fg3);
+}
+
 .dlayout {
   display: flex;
   align-items: stretch;