refactor(admin): remove download config settings page and chain

- Drop the download config menu entry, route and schema from the settings SPA
- Remove the PHP schema section, the server-side alias validation and the
  reserved-alias list that only served that section
- Unregister aksrt_download_config from standalone import/export keys
- Drop the contract tests that locked the removed page; the Download.php
  runtime contracts stay untouched

src/Settings/Settings.php

@@ -36,7 +36,6 @@ class Settings {
     public const STANDALONE_OPTION_KEYS = [
         'aksrt_captcha_config',
         'aksrt_moderation_config',
-        'aksrt_download_config',
         'aksrt_footer_tabbar',
         'aksrt_floating_buttons',
         'aksrt_credit_settings',