refactor: ES6+ modernization + 2 bugfixes

- var→const/let, function→arrow/class across all 13 JS files
- Timer: prototype→class with ES6 getters
- plan days: IIFE→Array.from declarative generation
- storage/plan: unified formatDate via util.js
- Bugfix: getPlanDay now filters by planId (plan switch accuracy)
- Bugfix: getTodayRecord searches all months (cross-month boundary)
- WXML/WXSS unchanged; public API unchanged

app.js

@@ -1,8 +1,8 @@
-var themeMod = require('./utils/theme')
+const themeMod = require('./utils/theme')
 
 App({
-  onLaunch: function () {
-    var settings = wx.getStorageSync('user_settings')
+  onLaunch() {
+    const settings = wx.getStorageSync('user_settings')
     if (!settings) {
       wx.setStorageSync('user_settings', {
         planId: 'beginner',
@@ -12,7 +12,7 @@ App({
         vibrate: true
       })
     }
-    var streak = wx.getStorageSync('current_streak')
+    const streak = wx.getStorageSync('current_streak')
     if (!streak) {
       wx.setStorageSync('current_streak', { count: 0, lastDate: '' })
     }