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。

pages/settings/settings.js

@@ -136,19 +136,34 @@ Page({
 
   // --- Profile handlers ---
 
-  onChooseAvatar(e) {
+  async onChooseAvatar(e) {
     const avatarUrl = e.detail.avatarUrl
     if (!avatarUrl) return
-    this.setData({ avatarUrl })
-    const saved = storage.saveProfile({
-      nickname: this.data.nickName,
-      avatarUrl
-    })
-    this.setData({
-      nickName: saved.nickname || this.data.nickName,
-      avatarUrl: saved.avatarUrl
-    })
-    wx.showToast({ title: '头像已更新', icon: 'success', duration: 1200 })
+    try {
+      // chooseAvatar returns a wxfile:// temporary URL that expires in a few
+      // days. Read the bytes and store as a data URI so the avatar survives
+      // long-term. Falls back to the raw URL on size/read error.
+      const permanentUrl = await util.fileToDataURI(avatarUrl)
+      this.setData({ avatarUrl: permanentUrl })
+      const saved = storage.saveProfile({
+        nickname: this.data.nickName,
+        avatarUrl: permanentUrl
+      })
+      this.setData({
+        nickName: saved.nickname || this.data.nickName,
+        avatarUrl: saved.avatarUrl
+      })
+      wx.showToast({ title: '头像已更新', icon: 'success', duration: 1200 })
+    } catch (err) {
+      console.warn('avatar convert failed, saving temp URL:', err)
+      // Fallback: save the raw wxfile URL. Will eventually expire.
+      this.setData({ avatarUrl })
+      storage.saveProfile({
+        nickname: this.data.nickName,
+        avatarUrl
+      })
+      wx.showToast({ title: '头像已更新', icon: 'success', duration: 1200 })
+    }
   },
 
   onNicknameBlur(e) {