Heading segments, story arc transitions, and life event sorting

Heading segments - books that are divided into parts (like Return of the King's Book V and Book VI) now display those structural divisions as labels in the character story arc. Curators can mark any segment as a heading, and heading segments are excluded from the appearance assignment picker.

Story arc transitions - expanding and collapsing series and book sections in the story arc now animates smoothly instead of snapping open and closed.

Life event ordering - the History table on universe and series pages now displays events in correct chronological order, accounting for in-universe dates across different calendar systems.

Blog tag suggestions - the blog post editor now shows all existing tags below the input field for quick selection, filtering as you type.