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/routes/lobby/+page.svelte

@@ -3,6 +3,7 @@
 	import { createEdgeGatewayClient } from "../../api/connect";
 	import { GalaxyClient } from "../../api/galaxy-client";
 	import { GATEWAY_BASE_URL, GATEWAY_RESPONSE_PUBLIC_KEY } from "$lib/env";
+	import { i18n } from "$lib/i18n/index.svelte";
 	import { loadCore } from "../../platform/core/index";
 	import { session } from "$lib/session-store.svelte";
 
@@ -64,22 +65,23 @@
 </script>
 
 <main>
-	<h1>you are logged in</h1>
+	<h1>{i18n.t("lobby.title")}</h1>
 	<p>
-		device session id: <code data-testid="device-session-id"
-			>{session.deviceSessionId ?? ""}</code
-		>
+		{i18n.t("lobby.device_session_id_label")}:
+		<code data-testid="device-session-id">{session.deviceSessionId ?? ""}</code>
 	</p>
 	{#if accountLoading}
-		<p>loading account…</p>
+		<p>{i18n.t("lobby.account_loading")}</p>
 	{:else if displayName !== null}
-		<p>
-			hello, <span data-testid="account-display-name">{displayName}</span>!
+		<p data-testid="account-greeting">
+			{i18n.t("lobby.greeting", { name: displayName })}
 		</p>
 	{:else if accountError !== null}
 		<p role="alert" data-testid="account-error">{accountError}</p>
 	{/if}
-	<button onclick={logout} data-testid="lobby-logout">logout</button>
+	<button onclick={logout} data-testid="lobby-logout">
+		{i18n.t("lobby.logout")}
+	</button>
 </main>
 
 <style>