Arrow-key navigation required a click to establish an active photo
first. Now the grid auto-selects photos[0] on initial mount when no
active photo is set, so the user can land on the app and immediately
walk the grid with arrows. Guarded on viewMode === 'grid' and
!activePhotoId so we never clobber an existing selection or fight
with PreviewView.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>