- fetchSessions merges into existing items (preserve object references)
  instead of wholesale array replacement — DynamicScroller reuses rows,
  the list no longer repaints/reflows on every refetch (measured: sidebar
  repaint spikes 8-17% per switch -> 0)
- AppSidebar.handleSelect no longer refetches the list on a plain select;
  refetch only when leaving search (the array still held search results)
- loadSession now lives only in handleSelect — SessionList.onSelect used
  to start a first load and handleSelect raced it with a second one
  (double fetch + double buildMessageList, incl. search-jump target)
- pinSession mutates in place instead of replacing every item object
- SessionItem.highlightText matches on the raw string so <mark> offsets
  stay correct when the text contains &/</>