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/view.go

@@ -672,6 +672,7 @@ a.btn, a.btn:visited { color: var(--accent-fg); }
 
 /* detail reader */
 .detail { max-width: 38rem; }
+.detail .back { margin: 0 0 var(--s3); font-size: .85rem; }
 .detail h1 { font-size: 1.7rem; line-height: 1.25; margin: 0 0 var(--s2); }
 .detail .meta { color: var(--muted); font-size: .9rem; margin: 0 0 var(--s2); display: flex; gap: var(--s2); align-items: center; flex-wrap: wrap; }
 .detail .source { margin: 0 0 var(--s4); font-size: .9rem; }