Add a timeline page grouping photos by day

New /timeline/ page renders photos grouped into consecutive same-day
runs (ISO date heading plus per-day count), reusing the square grid.
groupByDay collapses the newest-first photo list; the page is not
paginated. Linked from the profile header.

templates/index.html

@@ -20,6 +20,7 @@
         <p class="handle">{{.Handle}}</p>
         <p class="pcount"><strong>{{.Total}}</strong> posts</p>
         <p class="pbio">{{.Bio}}{{if .BioAlt}}<br><span class="ml">{{.BioAlt}}</span>{{end}}</p>
+        <a href="/timeline/" class="rss-a">Timeline</a>
         <a href="/feed.xml" class="rss-a">
           <svg width="14" height="14" viewBox="0 0 14 14" fill="currentColor" aria-hidden="true">
             <circle cx="2.5" cy="11.5" r="1.5"/>