Complete client UI i18n migration

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

packages/app/src/components/message-compaction-label.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 
+import { i18n } from "@/i18n/i18next";
 import { getCompactionMarkerLabel } from "./message-compaction-label";
 
 describe("getCompactionMarkerLabel", () => {
@@ -16,4 +17,13 @@ describe("getCompactionMarkerLabel", () => {
     );
     expect(getCompactionMarkerLabel({ status: "completed" })).toBe("Context compacted");
   });
+
+  it("renders labels in the active app language", async () => {
+    await i18n.changeLanguage("zh-CN");
+    try {
+      expect(getCompactionMarkerLabel({ status: "loading" })).toBe("正在压缩...");
+    } finally {
+      await i18n.changeLanguage("en");
+    }
+  });
 });