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/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">