Extend the integration suite (now 22 tests) to cover:
- All Mail view (04): single-account merge of Inbox + custom folders with
Junk excluded, and cross-account aggregation across every logged-in account.
- Message actions from the list context menu (05): mark read/unread, delete
(→ Trash), mark-as-spam (→ Junk) and not-spam round-trip, verified on both
the UI counters/row state and the server mailbox the message ends up in.
- Shared/delegated folders (06): a delegated folder (+ Trash/Junk) shared
alice→carol; the shared folder renders with its counter, and read/unread/
delete/spam performed there land correctly (server-verified).
Hooks added: data-testid on context-menu delete/spam/read-unread items
(via a testId prop on ContextMenuItem), data-shared on folder rows, and
testId/data-expanded on sidebar section headers to drive the Shared section.
Observations surfaced by the suite (asserted server-side / with a reconcile):
- mark-as-spam doesn't optimistically decrement the *source* counter the way
delete does; a visibility reconcile settles it.
- shared *destination* counters (shared Trash/Junk) don't refresh live —
forceSync reconciles the active account only, not shared accounts.