feat: cloud sync with WeChat CloudBase — data survives app deletion

Problem: All data stored in wx.StorageSync is permanently lost when
user deletes the mini-program or clears cache.

Solution: Sync data to WeChat CloudBase (wx.cloud.database).
- Graceful degradation: if cloud env isn't configured, all local
  storage works exactly as before.
- On app launch: if local records are empty, pull from cloud.
- On writes: push to cloud (debounced 2s) after saveRecord,
  deleteRecord, saveSettings, updateStreak, setTheme.
- Clear data in settings also clears cloud.

Files:
- utils/cloud.js: cloud sync wrapper (init, pullAll, pushAll, clearAll)
- app.js: cloud.init() + restore on fresh install
- utils/storage.js: cloud.pushAll() after every mutation
- utils/theme.js: cloud.pushAll() after setTheme
- pages/settings/settings.js: cloud.clearAll() when clearing data
- project.config.json: cloudfunctionRoot added

Setup required (one-time in WeChat DevTools):
  1. Open 'Cloud Development' panel → enable CloudBase
  2. Create environment → copy env ID
  3. In app.js, change wx.cloud.init() → wx.cloud.init({ env: 'YOUR-ENV-ID' })
  4. Create 'plank_data' collection in cloud database
  5. Set collection permission to 'Only creator can read/write'

app.js

@@ -1,7 +1,11 @@
 const themeMod = require('./utils/theme')
+const cloud = require('./utils/cloud')
 
 App({
   onLaunch() {
+    // Init cloud storage (no-op if not configured)
+    cloud.init()
+
     const settings = wx.getStorageSync('user_settings')
     if (!settings) {
       wx.setStorageSync('user_settings', {
@@ -12,11 +16,37 @@ App({
         vibrate: true
       })
     }
+
     const streak = wx.getStorageSync('current_streak')
     if (!streak) {
       wx.setStorageSync('current_streak', { count: 0, lastDate: '' })
     }
+
     this.globalData.theme = themeMod.getCurrentTheme()
+
+    // If local records are empty, try pulling from cloud
+    const records = wx.getStorageSync('training_records')
+    if (!records || Object.keys(records).length === 0) {
+      cloud.pullAll().then((cloudData) => {
+        if (!cloudData) return
+        try {
+          if (cloudData.records && Object.keys(cloudData.records).length > 0) {
+            wx.setStorageSync('training_records', cloudData.records)
+          }
+          if (cloudData.settings) {
+            wx.setStorageSync('user_settings', cloudData.settings)
+          }
+          if (cloudData.streak) {
+            wx.setStorageSync('current_streak', cloudData.streak)
+          }
+          if (cloudData.themeId) {
+            themeMod.setTheme(cloudData.themeId)
+          }
+        } catch (e) {
+          console.error('Cloud restore failed:', e)
+        }
+      })
+    }
   },
 
   globalData: {