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>

ui/frontend/src/lib/i18n/locales/ru.ts

@@ -0,0 +1,41 @@
+// Russian translation dictionary. The keys are identical to the
+// English dictionary in `en.ts`; the values are the human Russian
+// text. Adding a new key requires updating every locale file in
+// this folder so the `t()` helper does not fall back to English.
+
+import type en from "./en";
+
+const ru: Record<keyof typeof en, string> = {
+	"common.language": "язык",
+	"common.loading": "загрузка…",
+	"common.browser_not_supported_title": "браузер не поддерживается",
+	"common.browser_not_supported_body":
+		"Galaxy требует поддержки Ed25519 в WebCrypto. См. список поддерживаемых браузеров.",
+
+	"login.title": "вход в Galaxy",
+	"login.email_label": "электронная почта",
+	"login.email_required": "адрес не должен быть пустым",
+	"login.send_code": "отправить код",
+	"login.sending": "отправляем…",
+	"login.code_label": "код",
+	"login.code_required": "код не должен быть пустым",
+	"login.code_sent_to": "код отправлен на {email}",
+	"login.verify": "подтвердить",
+	"login.verifying": "проверяем…",
+	"login.send_new_code": "отправить новый код",
+	"login.change_email": "изменить адрес",
+	"login.challenge_expired":
+		"запрос устарел, запросите новый код",
+	"login.code_expired_or_used":
+		"код устарел или уже использован, запросите новый",
+	"login.device_key_not_ready":
+		"ключ устройства ещё не готов, перезагрузите страницу",
+
+	"lobby.title": "вы вошли в систему",
+	"lobby.device_session_id_label": "идентификатор сессии устройства",
+	"lobby.greeting": "здравствуйте, {name}!",
+	"lobby.account_loading": "загрузка профиля…",
+	"lobby.logout": "выйти",
+};
+
+export default ru;