refactor(app): modularize UI and usage logic

src/modules/language.js

@@ -0,0 +1,26 @@
+export const languageModule = {
+    setupLanguageSwitcher() {
+        const loginToggle = document.getElementById('language-toggle');
+        const mainToggle = document.getElementById('language-toggle-main');
+
+        if (loginToggle) {
+            loginToggle.addEventListener('click', () => this.toggleLanguage());
+        }
+        if (mainToggle) {
+            mainToggle.addEventListener('click', () => this.toggleLanguage());
+        }
+    },
+
+    toggleLanguage() {
+        const currentLang = i18n.currentLanguage;
+        const newLang = currentLang === 'zh-CN' ? 'en-US' : 'zh-CN';
+        i18n.setLanguage(newLang);
+
+        this.updateThemeButtons();
+        this.updateConnectionStatus();
+
+        if (this.isLoggedIn && this.isConnected) {
+            this.loadAllData(true);
+        }
+    }
+};