test(integration): All Mail, message actions, and shared-folder sync

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.

components/ui/context-menu.tsx

@@ -108,6 +108,8 @@ interface ContextMenuItemProps {
   disabled?: boolean;
   destructive?: boolean;
   shortcut?: string;
+  /** Stable hook for integration tests (not user-visible). */
+  testId?: string;
 }
 
 export function ContextMenuItem({
@@ -117,10 +119,12 @@ export function ContextMenuItem({
   disabled = false,
   destructive = false,
   shortcut,
+  testId,
 }: ContextMenuItemProps) {
   return (
     <button
       role="menuitem"
+      data-testid={testId}
       disabled={disabled}
       className={cn(
         "w-full px-3 py-1.5 text-sm text-start flex items-center gap-2",