You notice the “Award-Winning Karaoke” parlor here, the smiling teens waving “Free Carwash for Jesus” placards there. But there’s no sign of Dick Nixon — usually termed our nation’s “most controversial president” — whose most flattering biographer to date concluded: “It is sometimes difficult to see where the cynical politician in him ends, and where the visionary side of his character begins.”Considering author Jonathan Aitken’s own reservations, could even Yorba Linda — the high-octane-Republican Orange County city of 60,000 that advertises its “Lush Life” — be ashamed of his birthplace? Not hardly: Turn around and come back, and there’s a huge notice for the Library and Birthplace right on the town’s eastern boundary.