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/App.svelte

@@ -18,6 +18,7 @@
   import CommsHub from './game/CommsHub.svelte';
   import Feedback from './screens/Feedback.svelte';
   import Blocked from './screens/Blocked.svelte';
+  import BootError from './screens/BootError.svelte';
 
   onMount(() => {
     void bootstrap();
@@ -83,6 +84,10 @@
   {#if !routeIsLobby}
     <div class="splash">{t('common.loading')}</div>
   {/if}
+{:else if app.bootError}
+  <!-- A Mini App launch that failed to authenticate (e.g. the backend was down mid-deploy):
+       show the retry screen instead of falling back to the web login. -->
+  <BootError />
 {:else if app.blocked}
   <Blocked />
 {:else}
@@ -123,7 +128,7 @@
 <StaleInviteModal />
 <WelcomeRedeemModal />
 
-{#if routeIsLobby && !app.splashDone && !app.blocked}
+{#if routeIsLobby && !app.splashDone && !app.blocked && !app.bootError}
   <Splash />
 {/if}