- article page: ← / → navigation across the whole material tree (including
between sections), sticky sidebar outline with the current article highlighted
- editing on chapter, section and article pages: rename, publish/hide, delete,
create section/article, move articles while deleting a section; forms return
to the page they were opened from (validated internal `back` parameter)
- article page opens the edit dialog directly via ?edit=1 (used by "Править"
links on section/chapter pages)
- import: optional sync flag — with it stale sections/articles of archived
chapters are deleted (previous behaviour), without it import only adds and
updates, so material edited in the UI is not overwritten
- student area: lab page and lab cards now show the defense date next to the
submission deadline
- fix SpEL crash on the student lab page (`and` with nullable strings)
- migration V5 adds published flag (default true) to chapters/sections/articles
- ContentService: tree(includeHidden) filters, treeAll(), findArticle(includeHidden),
set*Published; public tree/article expose only published items
- admin API: GET /tree/all, GET /all/{chapter}/{section}/{article}, PUT
/{chapters|sections|articles}/{id}/published
- web: learn pages for admins load the full tree, show "скрыто" badges and
publish/hide buttons for every chapter, section and article; hidden articles
open in preview for admins, guests get "not found"; article view carries
published flag
