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

components/calendar-heatmap/calendar-heatmap.js

@@ -1,4 +1,4 @@
-var util = require('../../utils/util')
+const util = require('../../utils/util')
 
 Component({
   properties: {
@@ -12,70 +12,63 @@ Component({
   },
 
   observers: {
-    'year, month, records': function (year, month, records) {
+    'year, month, records'(year, month, records) {
       this._compute(year, month, records)
     }
   },
 
   lifetimes: {
-    ready: function () {
-      this._compute(this.properties.year, this.properties.month, this.properties.records)
+    ready() {
+      const { year, month, records } = this.properties
+      this._compute(year, month, records)
     }
   },
 
   methods: {
-    _compute: function (year, month, records) {
-      var weeks = util.getMonthCalendar(year, month)
-      var recordMap = {}
+    _compute(year, month, records) {
+      const weeks = util.getMonthCalendar(year, month)
+      const recordMap = {}
       if (records && records.length) {
-        records.forEach(function (r) {
-          var d = parseInt(r.date.split('-')[2])
+        records.forEach((r) => {
+          const d = parseInt(r.date.split('-')[2])
           recordMap[d] = (recordMap[d] || 0) + r.duration
         })
       }
 
-      var data = weeks.map(function (week) {
-        return week.map(function (day) {
+      const data = weeks.map((week) =>
+        week.map((day) => {
           if (!day) return null
-          var duration = recordMap[day] || 0
-          var color = 'transparent'
+          const duration = recordMap[day] || 0
+          let color = 'transparent'
           if (duration > 0) {
             if (duration < 30) color = '#FFE0D0'
             else if (duration < 60) color = '#FFB088'
             else if (duration < 120) color = '#FF6B35'
             else color = '#E05520'
           }
-          return { day: day, duration: duration, color: color }
+          return { day, duration, color }
         })
-      })
+      )
 
       this.setData({ weeks: data })
     },
 
-    onDayTap: function (e) {
-      var day = e.currentTarget.dataset.day
+    onDayTap(e) {
+      const day = e.currentTarget.dataset.day
       if (!day) return
-      var cell = null
-      var weeks = this.data.weeks
-      for (var i = 0; i < weeks.length; i++) {
-        for (var j = 0; j < weeks[i].length; j++) {
-          var c = weeks[i][j]
-          if (c && c.day === day) { cell = c; break }
-        }
-        if (cell) break
-      }
+      const cell = this.data.weeks.flat().find(c => c && c.day === day)
       if (!cell || cell.duration <= 0) return
       this.triggerEvent('daytap', {
         year: this.properties.year,
         month: this.properties.month,
-        day: day,
+        day,
         duration: cell.duration
       })
     },
 
-    getHeatColor: function (cell) {
+    getHeatColor(cell) {
       if (!cell || cell.duration <= 0) return 'transparent'
-      var d = cell.duration
+      const d = cell.duration
       if (d < 30) return '#FFE0D0'
       if (d < 60) return '#FFB088'
       if (d < 120) return '#FF6B35'