Add link previews

internal/server/server.go

@@ -245,6 +245,10 @@ type page struct {
 	// The reading measure that suits the upload page is far too narrow for a
 	// listing, which otherwise ends up behind a horizontal scrollbar.
 	Wide bool
+
+	// Preview is the link card for this page, nil when there is nothing
+	// sensible to tell somebody who has only been handed the URL.
+	Preview *preview
 }
 
 // page builds the common fields, resolving the session so the header can show