feat(web): add a back link to the detail page

The summary detail page only returned to the list via the brand logo. Add an
explicit "← Summaries" link at the top (UX review C3).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/views.templ

@@ -321,6 +321,7 @@ templ processingCard(r store.SummaryRow) {
 templ DetailPage(r store.SummaryRow) {
 	@Layout("Tapir — " + displayTitle(r)) {
 		<article class="detail">
+			<p class="back"><a href="/">← Summaries</a></p>
 			<h1>{ displayTitle(r) }</h1>
 			<p class="meta">
 				if detailMeta(r) != "" {