The Shins’ best music has always been bittersweet: lovely, floating melodies and airy arrangements tinged with a palpable, nostalgic sadness. It’s no secret why the band was handpicked as the musical highlight of the “isn’t life (painfully) beautiful?” romp that was Garden State. At the core of all those vibes is James Mercer, The Shins’ singer, songwriter and only truly steady…