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/mock/client.ts

@@ -136,7 +136,10 @@ export class MockGateway implements GatewayClient {
   }
 
   // --- auth ---
-  async authTelegram(): Promise<Session> {
+  async authTelegram(initData: string): Promise<Session> {
+    // e2e hook: an initData carrying this sentinel simulates a backend that rejects the launch,
+    // so the Mini App boot-failure path (silent retries → boot-error screen) can be exercised.
+    if (initData.includes('bootfail')) throw new GatewayError('unavailable');
     return { ...SESSION, isGuest: false };
   }
   async authGuest(): Promise<Session> {