fixed problem with non-unique React key when a strand has a 5' and a …

…3' extension adjacent to the same domain

lib/src/view/design_main_dna_sequence.dart

@@ -244,7 +244,7 @@ class DesignMainDNASequenceComponent extends UiComponent2<DesignMainDNASequenceP
       ..startOffset = start_offset
       ..style = style_map);
     return (Dom.text()
-      ..key = 'extension-dna-'
+      ..key = 'extension-dna-${ext.is_5p ? "5'" : "3'"}'
           'H${ext.adjacent_domain.helix},${ext.adjacent_domain.start}-${ext.adjacent_domain.end}'
       ..dy = dy)(text_path_props(subseq));
   }