refactor(iter-v2): T02 code quality - shared components, transactions, utils

- C2: ColorPicker and EditModal reusable components extracted
- C2: tag-manage and category-manage pages use EditModal (remove ~120 dup lines)
- C3: Upload logic extracted to server/src/utils/upload.ts
- C4: /recurring/sync wrapped in DB transaction
- C7: Backup download uses API_BASE (no hardcoded URLs)
- C9: getAvatarUrl() utility function, used in 5 files
- C1: getCurrentMonth() documented as aligned with server date.ts

client/src/stores/user.ts

@@ -1,7 +1,7 @@
 import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import * as api from '@/api/user'
-import { API_BASE } from '@/config'
+import { getAvatarUrl } from '@/utils/avatar'
 
 export const useUserStore = defineStore('user', () => {
   const userInfo = ref<api.UserInfo | null>(null)
@@ -16,11 +16,7 @@ export const useUserStore = defineStore('user', () => {
 
   const avatarUrl = computed(() => {
     const filename = userInfo.value?.avatar_url || uni.getStorageSync('xc:avatar_url') || ''
-    if (!filename) return ''
-    // 已是完整 URL（http/blob）直接返回
-    if (filename.startsWith('http') || filename.startsWith('blob:')) return filename
-    // 拼接：API_BASE + /user/avatar/ + 文件名
-    return `${API_BASE}/user/avatar/${filename}`
+    return getAvatarUrl(filename)
   })
 
   async function fetchUserInfo() {