feat(index): 首页随机姿势提示 + 移除冗余文案

- homeTip 从 config.postureTips 随机取一条，每次进首页换一条
- 移除圆环下方"平板支撑"单位文字，调整 ring 间距

pages/index/index.wxml

@@ -64,7 +64,6 @@
         <view class="target-ring">
           <text class="target-time" style="font-size: {{targetTimeFontSize}}rpx;">{{todayTargetText}}</text>
         </view>
-        <text class="target-unit">平板支撑</text>
       </view>
       <view class="today-status">
         <block wx:if="{{todayDone}}">
@@ -106,7 +105,7 @@
   <ui-card variant="soft" padded="{{false}}">
     <view class="tip-card">
       <image class="tip-icon" src="{{icons.lightFill}}" mode="aspectFit"></image>
-      <text class="tip-text">保持身体成一条直线，核心收紧，均匀呼吸</text>
+      <text class="tip-text">{{homeTip}}</text>
     </view>
   </ui-card>