Add slideshow: root-level, shuffle, fullscreen, click-to-album

- Root slideshow: all_media_entries walks the full media tree so
  /slideshow/ shows every photo across all albums; Slideshow button
  always appears on the root album page
- Shuffle and Full screen checkboxes sit next to the Slideshow button
  on the album page; options pass as ?shuffle=1&fullscreen=1 URL params
- Fullscreen uses a tap-to-activate overlay (browsers block auto-entry
  on page load); webkit-prefixed for Safari; ⛶ button and F key for
  mid-session toggle
- Fullscreen mode hides controls, counter, caption bar, and site header
- Exiting fullscreen auto-pauses so the current photo stays visible
- Click/tap anywhere in the stage navigates to the photo's album
  lightbox; reads the live src attribute instead of ssIdx to avoid a
  race where ssIdx advances during the cross-fade while the old photo
  is still on screen
- layout.erb excluded from slideshow (layout: false) so the site header
  never appears there
- CSS cache-busted with ?v=2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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