Nest child sessions (by parentID) under their parent in the mobile sessions
list, with a chevron in the row's left gutter to expand/collapse them
recursively. Top-level pagination counts only parent sessions; children whose
parent isn't in the same bucket stay top-level so nothing is hidden.
Expansion state lives in an in-memory store that survives closing and
reopening the sheet but resets on a full page reload.
Bohdan Triapitsyn
61765ca32c