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'

utils/storage.js

@@ -1,4 +1,5 @@
 const { formatDate } = require('./util')
+const cloud = require('./cloud')
 
 const RECORDS_KEY = 'training_records'
 const SETTINGS_KEY = 'user_settings'
@@ -14,6 +15,7 @@ const saveRecord = (record) => {
   records[month].push(record)
   records[month].sort((a, b) => b.date.localeCompare(a.date))
   wx.setStorageSync(RECORDS_KEY, records)
+  cloud.pushAll()
 }
 
 const deleteRecord = (recordId) => {
@@ -23,6 +25,7 @@ const deleteRecord = (recordId) => {
     if (records[month].length === 0) delete records[month]
   })
   wx.setStorageSync(RECORDS_KEY, records)
+  cloud.pushAll()
 }
 
 const getRecordsByMonth = (month) => {
@@ -55,6 +58,7 @@ const getSettings = () => wx.getStorageSync(SETTINGS_KEY) || {
 
 const saveSettings = (settings) => {
   wx.setStorageSync(SETTINGS_KEY, settings)
+  cloud.pushAll()
 }
 
 const getStreak = () => wx.getStorageSync(STREAK_KEY) || { count: 0, lastDate: '' }
@@ -73,6 +77,7 @@ const updateStreak = (date) => {
   }
   streak.lastDate = today
   wx.setStorageSync(STREAK_KEY, streak)
+  cloud.pushAll()
   return streak
 }