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/index.svelte.ts

@@ -0,0 +1,150 @@
+// Lightweight i18n primitive used by the login form, the root
+// layout, and the lobby placeholder. The translation table is a
+// per-locale flat dictionary keyed by dotted strings; lookup falls
+// back to the default (English) locale when a key is missing.
+//
+// Adding a new language is a two-step change inside this folder:
+//   1. drop a `locales/<bcp47>.ts` file mirroring the shape of
+//      `locales/en.ts` (TypeScript enforces matching keys via the
+//      `Record<keyof typeof en, string>` annotation in `ru.ts`);
+//   2. register the file in `SUPPORTED_LOCALES` below — that single
+//      list drives the language picker UI and the runtime lookup
+//      table at the same time.
+//
+// The locale state is exposed through a Svelte 5 runes singleton
+// (`i18n`) so components stay reactive without ceremony:
+// `<p>{i18n.t('login.title')}</p>` re-renders whenever
+// `i18n.locale` changes. Phase 35 will swap this primitive for a
+// fuller solution once message-format pluralisation and lazy
+// loading become necessary.
+
+import enTranslations from "./locales/en";
+import ruTranslations from "./locales/ru";
+
+export type Locale = "en" | "ru";
+export type TranslationKey = keyof typeof enTranslations;
+
+export interface LocaleEntry {
+	readonly code: Locale;
+	readonly nativeName: string;
+	readonly translations: Readonly<Record<TranslationKey, string>>;
+}
+
+export const SUPPORTED_LOCALES: readonly LocaleEntry[] = [
+	{
+		code: "en",
+		nativeName: "English",
+		translations: enTranslations,
+	},
+	{
+		code: "ru",
+		nativeName: "Русский",
+		translations: ruTranslations,
+	},
+];
+
+export const DEFAULT_LOCALE: Locale = "en";
+
+const TRANSLATIONS_BY_LOCALE: Record<
+	Locale,
+	Readonly<Record<TranslationKey, string>>
+> = SUPPORTED_LOCALES.reduce(
+	(acc, entry) => {
+		acc[entry.code] = entry.translations;
+		return acc;
+	},
+	{} as Record<Locale, Readonly<Record<TranslationKey, string>>>,
+);
+
+/**
+ * detectInitialLocale returns the best supported locale match for
+ * the supplied BCP 47 preference list. The web target passes
+ * `navigator.languages`; native wrappers pass the system locale
+ * (one entry). The first preference whose primary subtag matches
+ * a `SUPPORTED_LOCALES` entry wins; otherwise [DEFAULT_LOCALE].
+ */
+export function detectInitialLocale(
+	preferences?: readonly string[],
+): Locale {
+	const prefs = preferences ?? readBrowserPreferences();
+	for (const tag of prefs) {
+		const primary = primarySubtag(tag);
+		if (primary === null) {
+			continue;
+		}
+		const found = SUPPORTED_LOCALES.find((entry) => entry.code === primary);
+		if (found !== undefined) {
+			return found.code;
+		}
+	}
+	return DEFAULT_LOCALE;
+}
+
+function readBrowserPreferences(): readonly string[] {
+	if (typeof navigator === "undefined") {
+		return [];
+	}
+	if (Array.isArray(navigator.languages) && navigator.languages.length > 0) {
+		return navigator.languages;
+	}
+	if (typeof navigator.language === "string" && navigator.language !== "") {
+		return [navigator.language];
+	}
+	return [];
+}
+
+function primarySubtag(tag: string): Locale | null {
+	const trimmed = tag.trim().toLowerCase();
+	if (trimmed.length === 0) {
+		return null;
+	}
+	const code = trimmed.split(/[-_]/)[0] ?? "";
+	return isLocale(code) ? code : null;
+}
+
+function isLocale(value: string): value is Locale {
+	return SUPPORTED_LOCALES.some((entry) => entry.code === value);
+}
+
+class I18nStore {
+	locale: Locale = $state(detectInitialLocale());
+
+	/**
+	 * setLocale changes the active locale. Components reading
+	 * `i18n.t(...)` re-render automatically through the rune.
+	 */
+	setLocale(next: Locale): void {
+		this.locale = next;
+	}
+
+	/**
+	 * t looks up `key` in the active locale, falling back to the
+	 * default locale when the key is missing. `params` is an optional
+	 * `{name -> value}` map; placeholders in the template (`{name}`)
+	 * are replaced literally with no escaping — callers are expected
+	 * to feed user-safe values.
+	 */
+	t(key: TranslationKey, params?: Record<string, string>): string {
+		const active = TRANSLATIONS_BY_LOCALE[this.locale];
+		const fallback = TRANSLATIONS_BY_LOCALE[DEFAULT_LOCALE];
+		const template = active[key] ?? fallback[key] ?? key;
+		if (params === undefined) {
+			return template;
+		}
+		return template.replace(/\{(\w+)\}/g, (match, name: string) => {
+			const value = params[name];
+			return value === undefined ? match : value;
+		});
+	}
+
+	/**
+	 * resetForTests forces the singleton back to its module-load
+	 * state. Production code never calls this; the Vitest harness
+	 * uses it to keep cases independent.
+	 */
+	resetForTests(initial: Locale = detectInitialLocale()): void {
+		this.locale = initial;
+	}
+}
+
+export const i18n = new I18nStore();