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).

docs/FUNCTIONAL.md

@@ -31,7 +31,10 @@ ephemeral guest. The gateway validates the credential once and mints a thin
 session token; the backend resolves it to an internal `user_id`. A **Telegram Mini
 App** launch authenticates from the platform's signed `initData`, themes the UI to
 the Telegram colours, and — on first contact — seeds the new account's interface
-language from the Telegram client. Telegram runs a **single bot**: every player uses
+language from the Telegram client. If a launch cannot reach the backend (for example during a
+deployment), the Mini App retries quietly and then shows a small "couldn't load" screen with a
+**Retry** button, rather than dropping to the web sign-in, which has no place inside Telegram.
+Telegram runs a **single bot**: every player uses
 the same bot, and all of its chat and out-of-app notifications are written in the
 player's own **interface language** (en/ru). A separate optional **promo bot** can run alongside the
 main one — its only job is to answer `/start` with a short message and a button that opens the
@@ -56,6 +59,10 @@ reconnect), and pending reads resume on their own — the interface stays usable
 flashing a red banner each time.
 
 ### Accounts, linking & merge
+_Sign-in is currently provider-only, so the in-profile linking UI is temporarily hidden; it
+returns once the anonymous `/app/` guest (whose upgrade path this is) ships. The flow below
+describes it for when it does._
+
 First platform contact auto-provisions a durable account. From the profile a player
 links an email (via a confirm code) or their Telegram (via the web sign-in); a guest
 who links their first identity becomes a durable account. The "already taken" status