fix: take a conversation's size from the list row's own account

The placeholder cards held for a loading conversation looked its size up
by bare thread id whenever the open folder was the message's account.
Rows of an "All folders" search or a tag view now come from several
accounts and the list keys their conversation sizes by account, so the
viewer looks the size up the same way, and only for pages that are the
list's own rows.

src/screens/EmailThreadScreen.tsx

@@ -42,6 +42,7 @@ import { pickEmailBody, plainTextBody } from '../lib/email-body';
 import { singleLine } from '../lib/single-line';
 import { buildForwardAsAttachmentPayload } from '../lib/forward-as-attachment';
 import { viewerInstance, viewerPages, type ViewerInstance } from '../lib/viewer-pages';
+import { accountScopedId } from '../lib/thread-utils';
 import type { Email, EmailAddress, Identity } from '../api/types';
 import type { RootStackParamList } from '../navigation/types';
 
@@ -218,13 +219,14 @@ function EmailViewer({ route, navigation }: Props) {
     if (view) return view.ids;
     return failedThreads.has(threadId) ? [] : null;
   }, [ownerAccountId, failedThreads]);
-  // The conversation's size as the open folder's list knows it (only when
-  // that list is the message's account: thread ids are per account).
+  // The conversation's size as the store's list knows it, for pages that are
+  // that list's rows (its counts are keyed by the row's account and thread).
+  const [storeRows] = React.useState(() => new Set(useEmailStore.getState().emails));
   const threadSizeOf = React.useCallback(
-    (threadId: string): number | undefined => (listAccountId === ownerAccountId
-      ? useEmailStore.getState().threadCounts[threadId]
+    (page: Email): number | undefined => (storeRows.has(page) && page.threadId
+      ? useEmailStore.getState().threadCounts[accountScopedId(page, page.threadId)]
       : undefined),
-    [listAccountId, ownerAccountId],
+    [storeRows],
   );
   const memberOf = React.useCallback(
     (threadId: string, id: string): Email | undefined =>
@@ -769,7 +771,7 @@ function EmailViewer({ route, navigation }: Props) {
                       : null
                   }
                   memberOf={memberOf}
-                  threadSizeHint={!disableThreading ? threadSizeOf(item.threadId) : undefined}
+                  threadSizeHint={!disableThreading ? threadSizeOf(item) : undefined}
                   threading={!disableThreading}
                   jmapAccountId={ownerAccountId}
                   currentMailboxRole={currentMailboxRole}