feat(settings): always show the Unified Mailbox switch in Layout settings

Drop the `accounts.length > 1 || hasGroupInboxes` gate that hid the
Unified Mailbox toggle for single-account users with no visible shared
folder. The admin `isSettingHidden('enableUnifiedMailbox')` policy gate
is preserved, so admins can still hide it.

components/settings/layout-settings.tsx

@@ -245,7 +245,7 @@ export function LayoutSettings() {
         />
       </SettingItem>
 
-      {(accounts.length > 1 || hasGroupInboxes) && !isSettingHidden('enableUnifiedMailbox') && (
+      {!isSettingHidden('enableUnifiedMailbox') && (
         <SettingItem
           label={t('unified_mailbox.label')}
           description={t('unified_mailbox.description')}