Gallery pages: title in header, remove hgroup, add back link

- Album title moved into the header bar (centered, small-caps)
- hgroup removed from page body via single.html and list.html overrides
- Back-to-home link added at the bottom of each gallery page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/_default/list.html

@@ -0,0 +1,7 @@
+{{ define "main" }}
+  <section class="galleries">
+    {{ range where .Pages "Params.private" "ne" true }}
+      {{ partial "album-card.html" . }}
+    {{ end }}
+  </section>
+{{ end }}