Normalize wallpaper filenames (no capitals, no spaces) and regenerate
the alphabetical gallery split for the new 88-wallpaper total; m-r/r-z
replace n-r/s-z since the split boundaries shifted.
Claude-Session: https://claude.ai/code/session_011kf23kB9NsaczSYazZsWs6
Move the wallpaper gallery docs into wallpaper-gallery/ (index +
alphabetical a-d/e-m/n-r/s-z pages) so the growing list stays readable,
and link the new structure from README.
Claude-Session: https://claude.ai/code/session_011kf23kB9NsaczSYazZsWs6