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/handlers_test.go

@@ -234,6 +234,7 @@ func TestDetailRendersHighlightsAndTakeaways(t *testing.T) {
 	require.Contains(t, html, "takeaway one")
 	require.Contains(t, html, `id="action-buttons"`, "action button group present")
 	require.Contains(t, html, "Watched")
+	require.Contains(t, html, "← Summaries", "back link to the list (UX review C3)")
 
 	// The detail page leads with the attention-saving payload (UX review A8):
 	// Takeaways ("is it worth my time?") above Highlights above the full Summary.