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/screens/BootError.svelte

@@ -0,0 +1,63 @@
+<script lang="ts">
+  // The Mini App launch failed to authenticate after its silent retries (e.g. the backend was
+  // briefly down during a deploy). Inside Telegram there is no web login to fall back to, so this
+  // terminal screen offers a manual Retry that re-runs the launch (app.svelte retryTelegramBoot).
+  import { retryTelegramBoot } from '../lib/app.svelte';
+  import { t } from '../lib/i18n/index.svelte';
+
+  let retrying = $state(false);
+  async function retry(): Promise<void> {
+    if (retrying) return;
+    retrying = true;
+    try {
+      await retryTelegramBoot();
+    } finally {
+      retrying = false;
+    }
+  }
+</script>
+
+<div class="boot">
+  <div class="card">
+    <h1>{t('boot.errorTitle')}</h1>
+    <p class="msg">{t('boot.errorBody')}</p>
+    <button class="retry" onclick={retry} disabled={retrying}>{t('common.retry')}</button>
+  </div>
+</div>
+
+<style>
+  .boot {
+    height: 100%;
+    display: grid;
+    place-items: center;
+    padding: 24px;
+    background: var(--bg);
+  }
+  .card {
+    max-width: 28rem;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    gap: 1rem;
+    text-align: center;
+    color: var(--text);
+  }
+  h1 {
+    margin: 0;
+    font-size: 1.25rem;
+  }
+  .msg {
+    margin: 0;
+    color: var(--text-muted);
+  }
+  .retry {
+    padding: 9px 16px;
+    border: 1px solid var(--accent);
+    background: var(--accent);
+    color: var(--accent-text);
+    border-radius: var(--radius-sm);
+  }
+  .retry:disabled {
+    opacity: 0.5;
+  }
+</style>

ui/src/screens/Profile.svelte

@@ -244,9 +244,10 @@
       </form>
     {/if}
 
-    <!-- Linking & merge. Shown to everyone, including guests, who
-         upgrade by binding their first identity. -->
-    <section class="emailbox">
+    <!-- Linking & merge. Hidden for now: we target provider sign-in, and the anonymous
+         /app/ guest (whose upgrade path this is) comes later. Kept wired — drop `hidden`
+         to re-enable, together with the skipped linking specs in e2e/social.spec.ts. -->
+    <section class="emailbox" hidden>
       <h3>{t('profile.linkAccount')}</h3>
       {#if !emailSent}
         <div class="addrow">