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/tests/i18n.test.ts

@@ -0,0 +1,107 @@
+// Unit tests for the lightweight i18n primitive in
+// `src/lib/i18n/index.svelte.ts`. The locale singleton is reset
+// between cases through `resetForTests`; the default constructor
+// derives the locale from JSDOM's `navigator.language`, so the
+// reset takes an explicit value when a case needs a deterministic
+// starting state.
+
+import { afterEach, beforeEach, describe, expect, test } from "vitest";
+import {
+	DEFAULT_LOCALE,
+	SUPPORTED_LOCALES,
+	detectInitialLocale,
+	i18n,
+	type Locale,
+} from "../src/lib/i18n/index.svelte";
+
+beforeEach(() => {
+	i18n.resetForTests("en");
+});
+
+afterEach(() => {
+	i18n.resetForTests("en");
+});
+
+describe("detectInitialLocale", () => {
+	test("matches the first supported primary subtag", () => {
+		expect(detectInitialLocale(["ru-RU", "en-US"])).toBe("ru");
+		expect(detectInitialLocale(["en-GB", "ru"])).toBe("en");
+		expect(detectInitialLocale(["RU"])).toBe("ru");
+	});
+
+	test("skips unsupported tags and falls back to the default locale", () => {
+		expect(detectInitialLocale(["fr-FR", "de-DE", "ja"])).toBe(DEFAULT_LOCALE);
+		expect(detectInitialLocale([])).toBe(DEFAULT_LOCALE);
+	});
+
+	test("ignores whitespace and casing", () => {
+		expect(detectInitialLocale(["  En-us  "])).toBe("en");
+		expect(detectInitialLocale(["RU_ru"])).toBe("ru");
+	});
+});
+
+describe("SUPPORTED_LOCALES", () => {
+	test("each entry exposes a code, native name and translation table", () => {
+		for (const entry of SUPPORTED_LOCALES) {
+			expect(entry.code).toMatch(/^[a-z]{2}$/);
+			expect(entry.nativeName.length).toBeGreaterThan(0);
+			expect(entry.translations["login.title"].length).toBeGreaterThan(0);
+		}
+	});
+
+	test("native names cover the two phase-7 locales", () => {
+		const codes = SUPPORTED_LOCALES.map((l) => l.code);
+		const names = SUPPORTED_LOCALES.map((l) => l.nativeName);
+		expect(codes).toEqual(["en", "ru"]);
+		expect(names).toEqual(["English", "Русский"]);
+	});
+});
+
+describe("i18n.t", () => {
+	test("returns the active locale's translation", () => {
+		i18n.setLocale("en");
+		expect(i18n.t("login.title")).toBe("sign in to Galaxy");
+		i18n.setLocale("ru");
+		expect(i18n.t("login.title")).toBe("вход в Galaxy");
+	});
+
+	test("returns the key itself for an unknown identifier", () => {
+		// `t` is typed `TranslationKey`, but bracket access at runtime
+		// gracefully handles unknown keys — the fallback chain tries
+		// the active locale, then the default locale, then the literal
+		// key. This is the safety net for a future locale that adds
+		// keys before another locale catches up.
+		i18n.setLocale("en");
+		expect(
+			i18n.t("does.not.exist" as unknown as Parameters<typeof i18n.t>[0]),
+		).toBe("does.not.exist");
+	});
+
+	test("interpolates {placeholder} parameters", () => {
+		i18n.setLocale("en");
+		expect(i18n.t("login.code_sent_to", { email: "x@y.z" })).toBe(
+			"code sent to x@y.z",
+		);
+		i18n.setLocale("ru");
+		expect(i18n.t("login.code_sent_to", { email: "x@y.z" })).toBe(
+			"код отправлен на x@y.z",
+		);
+	});
+
+	test("leaves unresolved placeholders intact", () => {
+		i18n.setLocale("en");
+		expect(i18n.t("login.code_sent_to")).toContain("{email}");
+		expect(i18n.t("login.code_sent_to", { other: "x" })).toContain(
+			"{email}",
+		);
+	});
+});
+
+describe("i18n.setLocale", () => {
+	test("setLocale updates the reactive state", () => {
+		i18n.setLocale("en");
+		expect(i18n.locale).toBe<Locale>("en");
+		i18n.setLocale("ru");
+		expect(i18n.locale).toBe<Locale>("ru");
+	});
+});