Add single-day pages linked from timeline date headings

Each timeline date heading now links to /day/<YYYY-MM-DD>/, a page
holding just that day's photos with a back link to the timeline.
renderDayPages writes one per groupByDay run; the ISO label doubles
as the URL slug.

static/style.css

@@ -217,6 +217,15 @@ body {
   border-bottom: 1px solid var(--line);
 }
 
+.tl-date a {
+  color: inherit;
+  text-decoration: none;
+}
+
+.tl-date a:hover {
+  color: var(--aqua);
+}
+
 .tl-count {
   font-family: monospace;
   font-size: 11px;