fix: theme not applying to training/records pages

Root cause: themeStyle was on .container only, but modals (picker,
day-detail) were siblings of .container in the WXML tree. CSS custom
properties set via inline style on .container don't cascade to
sibling elements — those elements fall back to app.wxss page-level
defaults (always orange). Users saw modal buttons in orange and
concluded the whole page didn't follow theme.

Fixes:
- index.wxml: Move picker modal inside .container so it inherits
  themeStyle CSS variables (position:fixed is viewport-relative,
  so nesting doesn't affect modal positioning)
- index.js, records.js: Add applyThemeToPage in onLoad alongside
  onShow, preventing a flash of default orange before onShow fires
- timer.js: Add onShow with applyThemeToPage as lifecycle safety
  net; extract _init from onLoad for clean separation

pages/index/index.js

@@ -23,6 +23,7 @@ Page({
   },
 
   onLoad() {
+    themeMod.applyThemeToPage(this)
     this.refresh()
   },