Complete client UI i18n migration

Adds client-side i18n support, language settings, translated UI copy, and locale parity coverage.

packages/app/src/review/surface.test.tsx

@@ -1,5 +1,6 @@
 // @vitest-environment jsdom
 import "@/test/window-local-storage";
+import { i18n as testI18n } from "@/i18n/i18next";
 import { act, fireEvent, render, renderHook, cleanup } from "@testing-library/react";
 import React from "react";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
@@ -19,6 +20,8 @@ import {
   type InlineReviewActions,
 } from "./index";
 
+void testI18n;
+
 const { theme, pressablePropsByLabel } = vi.hoisted(() => {
   Object.assign(globalThis, { __DEV__: false });
   return {