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/email/email-context-menu.tsx

@@ -295,6 +295,7 @@ export function EmailContextMenu({
       <ContextMenuItem
         icon={Trash2}
         label={t("delete")}
+        testId="ctx-delete"
         onClick={() =>
           handleAction(showBatchActions ? onBatchDelete! : onDelete!)
         }
@@ -410,6 +411,7 @@ export function EmailContextMenu({
           <ContextMenuItem
             icon={isInJunkFolder ? ShieldCheck : ShieldAlert}
             label={isInJunkFolder ? t("not_spam") : t("mark_as_spam")}
+            testId={isInJunkFolder ? "ctx-not-spam" : "ctx-spam"}
             onClick={() =>
               handleAction(
                 showBatchActions
@@ -429,6 +431,7 @@ export function EmailContextMenu({
       <ContextMenuItem
         icon={isUnread ? MailOpen : Mail}
         label={isUnread ? t("mark_read") : t("mark_unread")}
+        testId={isUnread ? "ctx-mark-read" : "ctx-mark-unread"}
         onClick={() =>
           handleAction(() =>
             showBatchActions