WASHINGTON — Angela Merkel’s farewell visit to the White House was shadowed Thursday by pressing issues as well as good will, as she prepared to sit down with Joe Biden to discuss differences over a major Russian pipeline and national views on China as a rising global power. The German chancellor, who is not seeking another term in September elections, is nearing the end of a political career that has spanned four American presidencies. Merkel started her day with a working breakfast with Vice P