phase 7+: i18n primitive + login language picker + autocomplete-off

Adds a minimal Svelte 5 i18n primitive (`src/lib/i18n/`) backing the
login form, the layout blocker page, and the lobby placeholder.
SUPPORTED_LOCALES drives both the picker and the runtime lookup;
adding a language is a two-step change inside `src/lib/i18n/`.

Login form gains a globe-icon language dropdown (English / Русский
in their native names), defaulting to navigator.languages with `en`
as the fallback. Switching the locale re-renders the form in place;
on submit, the locale rides in the JSON body of `send-email-code`
because Safari/WebKit silently drops JS-set Accept-Language. Gateway
gains a body `locale` field that takes priority over the request
header for preferred-language resolution.

Email and code inputs disable browser autofill / suggestions
(`autocomplete=off` + `autocorrect=off` + `autocapitalize=off` +
`spellcheck=false`) so Keychain / address-book pickers and
remembered-value dropdowns no longer fire on focus.

Cross-cuts:
- backend & gateway openapi: clarify that body `locale` is honored.
- docs/FUNCTIONAL{,_ru}.md §1.2: document body-vs-header priority.
- gateway tests: body `locale` overrides Accept-Language; blank
  body `locale` falls back to header.
- new ui/docs/i18n.md; cross-links from auth-flow.md and ui/README.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/FUNCTIONAL.md

@@ -100,12 +100,15 @@ Branches inside backend:
   new one. The client gets the same response shape and is unaware of
   the reuse.
 - **Otherwise.** Backend creates a new challenge with the resolved
-  preferred language (derived from the optional `Accept-Language`
-  header forwarded by gateway, falling back to a default), and
-  enqueues the auth-mail row directly into the outbox in the same
-  transaction. SMTP delivery is asynchronous; the auth response
-  returns as soon as the challenge and outbox rows are durably
-  committed.
+  preferred language (derived from the optional `locale` body field
+  the caller sends — which takes priority — or, if absent or blank,
+  from the `Accept-Language` header forwarded by gateway, falling
+  back to a default), and enqueues the auth-mail row directly into
+  the outbox in the same transaction. SMTP delivery is asynchronous;
+  the auth response returns as soon as the challenge and outbox rows
+  are durably committed. The body field is the canonical channel
+  because Safari silently drops JS-set `Accept-Language` headers;
+  non-Safari clients can still rely on the header alone.
 
 ### 1.3 Confirming the challenge