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/en.ts

@@ -0,0 +1,40 @@
+// English translation dictionary. Keys are dotted strings grouped
+// by feature area (`login.*`, `lobby.*`, `common.*`); values are
+// the user-visible text. Adding a new key here also requires adding
+// it to every other locale dictionary in this folder, otherwise the
+// `t()` helper falls back to English at runtime.
+
+const en = {
+	"common.language": "language",
+	"common.loading": "loading…",
+	"common.browser_not_supported_title": "browser not supported",
+	"common.browser_not_supported_body":
+		"Galaxy requires Ed25519 in WebCrypto. See supported browsers.",
+
+	"login.title": "sign in to Galaxy",
+	"login.email_label": "email",
+	"login.email_required": "email must not be empty",
+	"login.send_code": "send code",
+	"login.sending": "sending…",
+	"login.code_label": "code",
+	"login.code_required": "code must not be empty",
+	"login.code_sent_to": "code sent to {email}",
+	"login.verify": "verify",
+	"login.verifying": "verifying…",
+	"login.send_new_code": "send a new code",
+	"login.change_email": "change email",
+	"login.challenge_expired":
+		"challenge expired, please request a new code",
+	"login.code_expired_or_used":
+		"code expired or already used, please request a new one",
+	"login.device_key_not_ready":
+		"device key is not ready, please reload the page",
+
+	"lobby.title": "you are logged in",
+	"lobby.device_session_id_label": "device session id",
+	"lobby.greeting": "hello, {name}!",
+	"lobby.account_loading": "loading account…",
+	"lobby.logout": "logout",
+} as const;
+
+export default en;