The view from the lifeguard’s chair ― bronzed faces bobbing in the water, toddlers packing sand into pails and dusting off seashells — takes in the easy fun of summer at the New England shore. But this summer, lifeguards have been confronted with a host of challenges that have turned the job into an anxious grind, scanning the horizon for danger that never seems far. The hottest June on record followed by a stretch of relentless rain in July. Unruly or even violent beachgoers. And a spate of drownings that have put even the most laidback guards on edge and heightened the pressure on a young workforce grappling with a severe staffing shortage.