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/+layout.svelte

@@ -2,6 +2,7 @@
 	import { onMount } from "svelte";
 	import { goto } from "$app/navigation";
 	import { page } from "$app/state";
+	import { i18n } from "$lib/i18n/index.svelte";
 	import { session } from "$lib/session-store.svelte";
 	import { startRevocationWatcher } from "$lib/revocation-watcher";
 
@@ -45,18 +46,12 @@
 
 {#if session.status === "loading"}
 	<main class="status">
-		<p>loading…</p>
+		<p>{i18n.t("common.loading")}</p>
 	</main>
 {:else if session.status === "unsupported"}
 	<main class="status">
-		<h1>browser not supported</h1>
-		<p>
-			Galaxy requires Ed25519 in WebCrypto. The minimum supported browser
-			versions are listed in the
-			<a href="https://github.com/galaxy/galaxy/blob/main/ui/docs/storage.md"
-				>storage topic doc</a
-			>.
-		</p>
+		<h1>{i18n.t("common.browser_not_supported_title")}</h1>
+		<p>{i18n.t("common.browser_not_supported_body")}</p>
 	</main>
 {:else}
 	{@render children()}