fix: drop a search folder scope that belongs to the account being left

`selectAccountMailbox` switches the viewed account but kept
`searchMailboxId`, which names a folder of the account being left.

Since #1082 that scope resolves against `resolveActionMailboxes()`, which
now returns the NEW account's mailbox list, so the id no longer matches
anything. `searchEmails` / `advancedSearch` then take the picked-folder
branch with `mailbox === undefined`, fall back to `jmapMailboxId =
searchMailboxId` and `accountId = undefined`, and query a foreign folder
id against the wrong account — which comes back empty.

It is invisible in the UI: the <select> binds `value={searchMailboxId}`
and has no matching <option>, so a native select renders the first one and
the dropdown reads "All folders" while the store still holds the stale id.
The user sees an empty result list for a search they believe is unscoped.

Reset it on the account switch, so the scope always matches what the
dropdown shows.

stores/__tests__/email-store-multi-account.test.ts

@@ -105,6 +105,19 @@ describe('useEmailStore multi-account state', () => {
     expect(state.expandedThreadIds.size).toBe(0);
   });
 
+  it('selectAccountMailbox drops a search scope belonging to the account being left', () => {
+    // The scope names a folder of the previous account and resolves against
+    // resolveActionMailboxes(), which now returns the new account's list. Kept,
+    // it would query that foreign folder id against the wrong account and come
+    // back empty, while the dropdown has no matching option and reads "All
+    // folders". (#1082)
+    useEmailStore.setState({ searchMailboxId: 'a-archive' });
+
+    useEmailStore.getState().selectAccountMailbox('account-b', 'b-inbox');
+
+    expect(useEmailStore.getState().searchMailboxId).toBe('');
+  });
+
   it('selectAccountMailbox with null accountId switches back to the active account', () => {
     useEmailStore.getState().selectAccountMailbox('account-b', 'b-inbox');
     expect(useEmailStore.getState().viewingAccountId).toBe('account-b');

stores/email-store.ts

@@ -1436,6 +1436,13 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
   selectAccountMailbox: (accountId, mailboxId) => set({
     viewingAccountId: accountId,
     selectedMailbox: mailboxId,
+    // The search panel's folder scope names a folder of the account we are
+    // leaving. It resolves against `resolveActionMailboxes()`, which now
+    // returns the NEW account's list, so the id no longer matches: the search
+    // would query that foreign folder id against the wrong account and come
+    // back empty, while the dropdown (which renders no matching option) reads
+    // "All folders". Reset it so the scope matches what is shown. (#1082)
+    searchMailboxId: "",
     isLoadingMore: false,
     selectedEmail: null,
     selectedEmailIds: new Set(),