refactor: 前端本地偏好统一为公共存储层

- 新增 frontend/src/utils/storageKeys.ts 集中维护存储键常量并登记旧键迁移
- 新增 frontend/src/utils/storage.ts 统一 localStorage/sessionStorage 读写与异常容错
- 新增 frontend/src/composables/usePreference.ts 提供 useStringPreference 响应式字符串偏好
- 新增 frontend/src/utils/transactionFilterPrefs.ts 抽出收支筛选偏好字段定义与共享读写
- 存储键统一以 finance. 前缀命名，locale、emailBill.tab、logs.tab 迁移到新键并自动迁移旧值
- 各页面与组件改用公共层，移除重复的 try/catch 与跨组件重复的序列化逻辑
- 认证凭据 token 属登录状态，保持原键名不纳入 finance. 前缀

frontend/src/locales/index.ts

@@ -1,6 +1,8 @@
 // 国际化入口：由对照消息构建各语言包，维护当前语言、持久化与 <html lang> 同步
 import { createI18n } from 'vue-i18n'
 import messages from './messages'
+import { migrateKey, readString, writeString } from '../utils/storage'
+import { LEGACY_STORAGE_KEYS, STORAGE_KEYS } from '../utils/storageKeys'
 
 /** 支持的语言标识 */
 export type LocaleKey = 'zh-CN' | 'en-US' | 'ja-JP'
@@ -13,7 +15,7 @@ interface MessageLeaf {
 }
 
 // 语言偏好存储键
-const STORAGE_KEY = 'locale'
+const STORAGE_KEY = STORAGE_KEYS.locale
 // 支持的语言列表
 const SUPPORTED: LocaleKey[] = ['zh-CN', 'en-US', 'ja-JP']
 
@@ -40,7 +42,9 @@ function buildLocaleMessages(node: unknown, lang: 'zh' | 'en' | 'ja'): any {
 
 /** 解析初始语言：localStorage > 浏览器语言 > 简体中文 */
 function resolveInitialLocale(): LocaleKey {
-  const saved = localStorage.getItem(STORAGE_KEY)
+  // 读取前迁移旧键 locale，保证老用户语言偏好不丢失
+  migrateKey(LEGACY_STORAGE_KEYS.locale, STORAGE_KEY)
+  const saved = readString(STORAGE_KEY)
   if (saved && SUPPORTED.includes(saved as LocaleKey)) {
     return saved as LocaleKey
   }
@@ -74,7 +78,7 @@ export function getLocale(): LocaleKey {
 /** 切换语言：更新 i18n、持久化到 localStorage，并同步 <html lang> */
 export function setLocale(locale: LocaleKey) {
   i18n.global.locale.value = locale
-  localStorage.setItem(STORAGE_KEY, locale)
+  writeString(STORAGE_KEY, locale)
   document.documentElement.lang = locale
 }