Fix race condition in useEffect hooks and type assertion bug (#2827)

- Add active flag pattern to 3 useEffect hooks in App.tsx to prevent
  event listener leaks when component unmounts before async setup completes
- Add guard check in useSettings.ts to prevent undefined from being
  stored in localStorage when payload.language is missing

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

src/hooks/useSettings.ts

@@ -14,8 +14,6 @@ import {
 } from "./useDirectorySettings";
 import { useSettingsMetadata } from "./useSettingsMetadata";
 
-type Language = "zh" | "en" | "ja";
-
 interface SaveResult {
   requiresRestart: boolean;
 }
@@ -402,11 +400,8 @@ export function useSettings(): UseSettingsResult {
         );
 
         try {
-          if (typeof window !== "undefined") {
-            window.localStorage.setItem(
-              "language",
-              payload.language as Language,
-            );
+          if (typeof window !== "undefined" && payload.language) {
+            window.localStorage.setItem("language", payload.language);
           }
         } catch (error) {
           console.warn(