fix: 头像转 base64 持久化

WeChat chooseAvatar 返回的 wxfile:// 临时 URL 几天后失效,
头像会变灰。加 util.fileToDataURI 把临时文件读成 data URI
存到本地(限制 512KB,超出走降级方案),从根上解决。

utils/util.js

@@ -54,11 +54,52 @@ const formatDate = (date) => {
 /** Strip the time portion of a formatDate string → "YYYY-MM-DD". */
 const _dateOnly = (s) => (typeof s === 'string' && s.length >= 10) ? s.substring(0, 10) : ''
 
+/**
+ * Convert a local file path (typically `wxfile://...` from chooseAvatar) to a
+ * base64 data URI. Used to make temporary file URLs survive the WeChat
+ * expiration window (a few days) by embedding the bytes inline.
+ *
+ * Rejects if the file exceeds `maxBytes` (default 512KB) so a stray huge
+ * image doesn't blow the 10MB total wx.storage budget. Caller should
+ * fall back to the raw URL on rejection.
+ */
+const fileToDataURI = (filePath, maxBytes = 512 * 1024) => {
+  return new Promise((resolve, reject) => {
+    if (!filePath || typeof wx.getFileSystemManager !== 'function') {
+      reject(new Error('FileSystemManager unavailable'))
+      return
+    }
+    const fs = wx.getFileSystemManager()
+    fs.getFileInfo({
+      filePath,
+      success: (info) => {
+        if (info.size > maxBytes) {
+          reject(new Error(`File too large: ${info.size} > ${maxBytes}`))
+          return
+        }
+        fs.readFile({
+          filePath,
+          encoding: 'base64',
+          success: (res) => {
+            // chooseAvatar returns PNG; allow JPG/GIF/WEBP fallbacks.
+            const m = (filePath.match(/\.(\w+)(?:\?|$)/) || [, 'png'])[1].toLowerCase()
+            const mime = { png: 'image/png', jpg: 'image/jpeg', jpeg: 'image/jpeg', gif: 'image/gif', webp: 'image/webp' }[m] || 'image/png'
+            resolve(`data:${mime};base64,${res.data}`)
+          },
+          fail: reject
+        })
+      },
+      fail: reject
+    })
+  })
+}
+
 module.exports = {
   formatTime,
   formatDuration,
   getDaysInMonth,
   getMonthCalendar,
   formatDate,
-  dateOnly: _dateOnly
+  dateOnly: _dateOnly,
+  fileToDataURI
 }