Complete client UI i18n migration

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

packages/app/e2e/helpers/project-settings.ts

@@ -34,7 +34,7 @@ export async function editWorktreeSetup(page: Page, setupCommands: string[]): Pr
 }
 
 export async function clickSaveProjectSettings(page: Page): Promise<void> {
-  await page.getByRole("button", { name: "Save project config" }).click();
+  await page.getByRole("button", { name: "Save" }).click();
 }
 
 export async function clickRetryProjectSettingsSave(page: Page): Promise<void> {
@@ -79,7 +79,7 @@ export async function expectWriteFailedCalloutActions(page: Page): Promise<void>
 }
 
 export async function expectSaveButtonDisabled(page: Page): Promise<void> {
-  await expect(page.getByRole("button", { name: "Save project config" })).toBeDisabled();
+  await expect(page.getByRole("button", { name: "Save" })).toBeDisabled();
 }
 
 // --- Form-state assertions ---

packages/app/e2e/settings-i18n.spec.ts

@@ -0,0 +1,23 @@
+import { test, expect } from "./fixtures";
+import { gotoAppShell, openSettings } from "./helpers/app";
+import { openSettingsSection } from "./helpers/settings";
+
+test("Settings language selector switches General labels", async ({ page }) => {
+  test.setTimeout(120_000);
+
+  await gotoAppShell(page);
+  await openSettings(page);
+  await openSettingsSection(page, "general");
+
+  await expect(page.getByText("Default send", { exact: true }).first()).toBeVisible();
+
+  await page.getByRole("button", { name: "System", exact: true }).click();
+  await page.getByRole("button", { name: "Simplified Chinese", exact: true }).click();
+
+  await expect(page.getByText("默认发送", { exact: true }).first()).toBeVisible();
+
+  await page.getByRole("button", { name: "简体中文", exact: true }).click();
+  await page.getByRole("button", { name: "English", exact: true }).click();
+
+  await expect(page.getByText("Default send", { exact: true }).first()).toBeVisible();
+});