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'

pages/settings/settings.js

@@ -112,6 +112,8 @@ Page({
         if (res.confirm) {
           wx.removeStorageSync('training_records')
           wx.removeStorageSync('current_streak')
+          // also clear cloud data
+          try { require('../../utils/cloud').clearAll() } catch (e) {}
           wx.showToast({ title: '已清除', icon: 'success', duration: 1500 })
         }
       }