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/e2e/auth-flow.spec.ts

@@ -143,7 +143,7 @@ test.describe("Phase 7 — auth flow", () => {
 		await expect(page.getByTestId("device-session-id")).toHaveText(
 			"dev-test-1",
 		);
-		await expect(page.getByTestId("account-display-name")).toHaveText("Pilot");
+		await expect(page.getByTestId("account-greeting")).toContainText("Pilot");
 
 		mocks.pendingSubscribes.forEach((resolve) => resolve());
 	});
@@ -153,7 +153,7 @@ test.describe("Phase 7 — auth flow", () => {
 	}) => {
 		const mocks = await mockGatewayHappyPath(page, "Pilot");
 		await completeLogin(page);
-		await expect(page.getByTestId("account-display-name")).toBeVisible();
+		await expect(page.getByTestId("account-greeting")).toBeVisible();
 
 		await page.reload();
 		await expect(page).toHaveURL(/\/lobby$/);
@@ -169,7 +169,7 @@ test.describe("Phase 7 — auth flow", () => {
 	}) => {
 		const mocks = await mockGatewayHappyPath(page, "Pilot");
 		await completeLogin(page);
-		await expect(page.getByTestId("account-display-name")).toBeVisible();
+		await expect(page.getByTestId("account-greeting")).toBeVisible();
 
 		// Fire all pending SubscribeEvents requests with an empty 200
 		// response. Connect-Web's server-streaming reader sees no frames
@@ -182,6 +182,48 @@ test.describe("Phase 7 — auth flow", () => {
 		expect(Date.now() - releaseAt).toBeLessThan(1500);
 	});
 
+	test("language picker switches the form text and forwards the locale", async ({
+		page,
+	}) => {
+		const sendRequests: Array<{ body: Record<string, unknown> }> = [];
+		await page.route(
+			"**/api/v1/public/auth/send-email-code",
+			async (route) => {
+				const raw = route.request().postData() ?? "";
+				sendRequests.push({
+					body: JSON.parse(raw) as Record<string, unknown>,
+				});
+				await route.fulfill({
+					status: 200,
+					contentType: "application/json",
+					body: JSON.stringify({ challenge_id: "ch-test-2" }),
+				});
+			},
+		);
+
+		await page.goto("/login");
+		await expect(page.getByTestId("login-email-submit")).toHaveText(
+			"send code",
+		);
+
+		await page
+			.getByTestId("login-language-select")
+			.selectOption("ru");
+		await expect(page.getByTestId("login-email-submit")).toHaveText(
+			"отправить код",
+		);
+
+		await page.getByTestId("login-email-input").fill("pilot@example.com");
+		await page.getByTestId("login-email-submit").click();
+		await expect(page.getByTestId("login-code-input")).toBeVisible();
+
+		expect(sendRequests).toHaveLength(1);
+		expect(sendRequests[0]!.body).toEqual({
+			email: "pilot@example.com",
+			locale: "ru",
+		});
+	});
+
 	test("browser without WebCrypto Ed25519 shows the not-supported blocker", async ({
 		page,
 	}) => {