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

custom-tab-bar/index.js

@@ -1,4 +1,4 @@
-var themeMod = require('../utils/theme')
+const themeMod = require('../utils/theme')
 
 Component({
   data: {
@@ -7,23 +7,23 @@ Component({
   },
 
   lifetimes: {
-    attached: function () {
-      var theme = themeMod.getCurrentTheme()
+    attached() {
+      const theme = themeMod.getCurrentTheme()
       this.setData({ themeStyle: themeMod.getThemeStyle(theme) })
     }
   },
 
   pageLifetimes: {
-    show: function () {
-      var theme = themeMod.getCurrentTheme()
+    show() {
+      const theme = themeMod.getCurrentTheme()
       this.setData({ themeStyle: themeMod.getThemeStyle(theme) })
     }
   },
 
   methods: {
-    switchTab: function (e) {
-      var index = e.currentTarget.dataset.index
-      var pages = [
+    switchTab(e) {
+      const index = e.currentTarget.dataset.index
+      const pages = [
         '/pages/index/index',
         '/pages/records/records',
         '/pages/settings/settings'