fix(ui): retry Mini App launch on backend failure; hide account linking

Inside Telegram, a failed initData authentication (e.g. the backend down
during a deploy) dropped the user onto the web login screen — the /app/
experience, which has no place inside the Mini App. bootstrap now retries the
launch a few times in silence and then renders a dedicated boot-error screen
with a Retry button (new BootError.svelte, app.bootError), never falling back
to the web sign-in. A blocked account is still terminal and goes straight to
the blocked screen.

The profile "Link an account" section (email + Telegram link) is hidden while
sign-in is provider-only; the anonymous /app/ guest whose upgrade path this is
comes later. The flow is kept wired (`hidden` on .emailbox) and its two e2e
specs are skipped, both to be re-enabled together.

Adds i18n boot.* copy (en/ru), a mock authTelegram failure hook plus an e2e
covering the retry screen, and bakes both behaviours into FUNCTIONAL(.md/_ru).

ui/src/lib/i18n/ru.ts

@@ -12,6 +12,9 @@ export const ru: Record<MessageKey, string> = {
   'blocked.temporary': 'Ваша учётная запись заблокирована до {until}.',
   'blocked.reason': 'Причина:',
 
+  'boot.errorTitle': 'Не удалось загрузить игру',
+  'boot.errorBody': 'Попробуйте ещё раз или зайдите позже.',
+
   'common.back': 'Назад',
   'common.cancel': 'Отмена',
   'common.ok': 'ОК',