feat: update language storage key and enhance theme button styles

src/i18n/index.ts

@@ -37,7 +37,7 @@ function buildResources() {
 // 获取持久化的语言设置（稍后由 Pinia store 提供）
 function getPersistedLanguage(): LanguageCode | null {
   try {
-    const saved = localStorage.getItem('diona-language');
+    const saved = localStorage.getItem('zn-language');
     return saved && SUPPORTED_LANGUAGE_CODES.includes(saved as LanguageCode) ? saved as LanguageCode : null;
   } catch {
     return null;
@@ -88,7 +88,7 @@ export async function setLanguage(lang: LanguageCode, _i18n?: I18n) {
   
   // 持久化到 localStorage（稍后由 Pinia store 处理）
   try {
-    localStorage.setItem('diona-language', lang);
+    localStorage.setItem('zn-language', lang);
   } catch {}
 }