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.wxml

@@ -76,39 +76,39 @@
     <text class="tip-icon">💡</text>
     <text class="tip-text">保持身体成一条直线，核心收紧，均匀呼吸</text>
   </view>
-</view>
 
-<!-- 时间选择弹窗 -->
-<view class="modal-overlay modal-overlay-bottom" wx:if="{{showPicker}}" bindtap="onClosePicker">
-  <view class="picker-modal" catchtap="noop" catchtouchmove="noop">
-    <view class="picker-handle"></view>
-    <text class="picker-title">⏱ 选择训练时长</text>
-    <view class="preset-grid">
-      <view
-        class="preset-item {{customDuration === item ? 'selected' : ''}}"
-        wx:for="{{presets}}"
-        wx:key="*this"
-        data-value="{{item}}"
-        bindtap="onSelectPreset"
-      >
-        <text class="preset-num">{{item}}</text>
-        <text class="preset-unit">秒</text>
+  <!-- 时间选择弹窗 -->
+  <view class="modal-overlay modal-overlay-bottom" wx:if="{{showPicker}}" bindtap="onClosePicker">
+    <view class="picker-modal" catchtap="noop" catchtouchmove="noop">
+      <view class="picker-handle"></view>
+      <text class="picker-title">⏱ 选择训练时长</text>
+      <view class="preset-grid">
+        <view
+          class="preset-item {{customDuration === item ? 'selected' : ''}}"
+          wx:for="{{presets}}"
+          wx:key="*this"
+          data-value="{{item}}"
+          bindtap="onSelectPreset"
+        >
+          <text class="preset-num">{{item}}</text>
+          <text class="preset-unit">秒</text>
+        </view>
       </view>
+      <view class="custom-input-row">
+        <text class="custom-label">自定义：</text>
+        <input
+          class="custom-input"
+          type="number"
+          placeholder="输入秒数"
+          value="{{customInput}}"
+          bindinput="onCustomInput"
+        />
+        <text class="custom-label">秒</text>
+      </view>
+      <button class="picker-confirm btn-primary" bindtap="onConfirmFree">
+        开始自由训练
+      </button>
+      <text class="picker-cancel" bindtap="onClosePicker">取消</text>
     </view>
-    <view class="custom-input-row">
-      <text class="custom-label">自定义：</text>
-      <input
-        class="custom-input"
-        type="number"
-        placeholder="输入秒数"
-        value="{{customInput}}"
-        bindinput="onCustomInput"
-      />
-      <text class="custom-label">秒</text>
-    </view>
-    <button class="picker-confirm btn-primary" bindtap="onConfirmFree">
-      开始自由训练
-    </button>
-    <text class="picker-cancel" bindtap="onClosePicker">取消</text>
   </view>
 </view>