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/cloud.js

@@ -0,0 +1,102 @@
+const DB_COLLECTION = 'plank_data'
+
+let _db = null
+let _pushTimer = null
+let _enabled = false
+
+const getDb = () => {
+  if (!_enabled) return null
+  if (!_db) {
+    try { _db = wx.cloud.database() } catch (e) { _enabled = false }
+  }
+  return _db
+}
+
+/**
+ * Call once from app.js onLaunch to initialize cloud.
+ * Gracefully degrades if cloud environment isn't configured yet.
+ */
+const init = () => {
+  try {
+    wx.cloud.init({ traceUser: true })
+    _enabled = true
+  } catch (e) {
+    _enabled = false
+  }
+}
+
+/**
+ * Pull all user data from cloud. Returns null if cloud is unavailable or no data exists.
+ */
+const pullAll = async () => {
+  const db = getDb()
+  if (!db) return null
+  try {
+    const coll = db.collection(DB_COLLECTION)
+    const res = await coll.where({ _openid: '{openid}' }).get()
+    if (res && res.data && res.data.length > 0) {
+      return res.data[0]
+    }
+    return null
+  } catch (e) {
+    console.error('Cloud pull failed:', e)
+    return null
+  }
+}
+
+/**
+ * Schedule a cloud push (debounced at 2s).
+ * Call this after any local data mutation.
+ */
+const pushAll = () => {
+  if (!_enabled) return
+  if (_pushTimer) clearTimeout(_pushTimer)
+  _pushTimer = setTimeout(() => _doPush(), 2000)
+}
+
+const _doPush = async () => {
+  const db = getDb()
+  if (!db) return
+  try {
+    // Lazily require to avoid circular dependency at module-load time
+    const storage = require('./storage')
+    const themeMod = require('./theme')
+
+    const data = {
+      records: storage.getRecords(),
+      settings: storage.getSettings(),
+      streak: storage.getStreak(),
+      themeId: themeMod.getCurrentTheme().id,
+      updatedAt: new Date()
+    }
+
+    const coll = db.collection(DB_COLLECTION)
+    const existing = await coll.where({ _openid: '{openid}' }).get()
+    if (existing && existing.data && existing.data.length > 0) {
+      await coll.doc(existing.data[0]._id).update({ data })
+    } else {
+      await coll.add({ data })
+    }
+  } catch (e) {
+    console.error('Cloud push failed:', e)
+  }
+}
+
+/**
+ * Delete all cloud data for current user.
+ */
+const clearAll = async () => {
+  const db = getDb()
+  if (!db) return
+  try {
+    const coll = db.collection(DB_COLLECTION)
+    const existing = await coll.where({ _openid: '{openid}' }).get()
+    if (existing && existing.data && existing.data.length > 0) {
+      await coll.doc(existing.data[0]._id).remove()
+    }
+  } catch (e) {
+    console.error('Cloud clear failed:', e)
+  }
+}
+
+module.exports = { init, pullAll, pushAll, clearAll, get enabled() { return _enabled } }