Complete client UI i18n migration

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

packages/app/src/hooks/use-settings/storage.test.ts

@@ -47,11 +47,20 @@ describe("loadAppSettingsFromStorage", () => {
     const result = await loadAppSettingsFromStorage(deps);
 
     expect(result).toEqual(DEFAULT_CLIENT_SETTINGS);
+    expect(DEFAULT_CLIENT_SETTINGS.language).toBe("system");
     expect(deps.storage.entries.get(APP_SETTINGS_KEY)).toBe(
       JSON.stringify(DEFAULT_CLIENT_SETTINGS),
     );
   });
 
+  it("defaults language to system when storage is empty", async () => {
+    const deps = makeDeps();
+
+    const result = await loadAppSettingsFromStorage(deps);
+
+    expect(result.language).toBe("system");
+  });
+
   it("loads configured terminal scrollback lines from app settings", async () => {
     const deps = makeDeps({
       storage: createInMemoryKeyValueStorage({
@@ -95,6 +104,30 @@ describe("loadAppSettingsFromStorage", () => {
     });
     expect(deps.storage.entries.get(APP_SETTINGS_KEY)).toBe(JSON.stringify(result));
   });
+
+  it("loads a persisted explicit language", async () => {
+    const deps = makeDeps({
+      storage: createInMemoryKeyValueStorage({
+        [APP_SETTINGS_KEY]: JSON.stringify({ language: "zh-CN" }),
+      }),
+    });
+
+    const result = await loadAppSettingsFromStorage(deps);
+
+    expect(result.language).toBe("zh-CN");
+  });
+
+  it("drops an unknown persisted language back to system", async () => {
+    const deps = makeDeps({
+      storage: createInMemoryKeyValueStorage({
+        [APP_SETTINGS_KEY]: JSON.stringify({ language: "klingon" }),
+      }),
+    });
+
+    const result = await loadAppSettingsFromStorage(deps);
+
+    expect(result.language).toBe("system");
+  });
 });
 
 describe("loadSettingsFromStorage", () => {