Atmospheric backgrounds, group fixes, and timeline tidying

Home page hero crossfade - the home page now has a slow, ambient slideshow of atmospheric imagery cycling behind the OpenFiction logo and search bar. Four images crossfade gently over a one-minute loop, each holding for around 12 seconds. A soft overlay keeps the text and search bar fully legible while letting the imagery show through at the edges. Respects your "Reduce motion" accessibility setting.

Group appearances fix - groups with only a single member appearing in a book (like The Ents in The Two Towers, where Treebeard is the sole on-page member) now correctly show that book in their appearances list. Previously the system required at least two members per book.

Story arc heading tidying - empty book/section labels in character story arcs are now hidden. If a character has no appearances within a particular structural section (e.g. Galadriel has no appearances in "Book V" of Return of the King), that heading no longer shows.