feat(fe): 冷启动过渡动画 — 爪印一步步踩上屏幕 + 收尾渐隐

- 启动登录已在 app.js onLaunch + store.ready 单例里最早触发，进首页不再等登录
- 闪屏把原来的单个 logo 换成左右交替的爪印小径，逐个延迟「踩」上屏走向 logo
- 登录/拉宠物完成后：先保证闪屏至少 900ms 让爪印走几步，再整屏渐隐放大切走，
  不再硬切到首页/建档向导

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pets-fe/pages/onboarding/onboarding.wxml

@@ -1,10 +1,12 @@
-<!-- 冷启动闪屏：登录 + 拉宠物期间显示，不再闪一下建档向导 -->
-<view wx:if="{{booting}}" class="boot">
-  <view class="boot-badge">
-    <view class="paw">
-      <view class="bean bean-1"></view><view class="bean bean-2"></view>
-      <view class="bean bean-3"></view><view class="bean bean-4"></view>
-      <view class="pad"></view>
+<!-- 冷启动闪屏：登录 + 拉宠物期间显示。一串爪印一步步「踩」上屏幕，走到 logo 那里 -->
+<view wx:if="{{booting}}" class="boot {{bootOut ? 'boot--out' : ''}}">
+  <view class="paw-trail">
+    <view wx:for="{{[0,1,2,3,4,5]}}" wx:key="*this" class="pw pw-{{item}}">
+      <view class="paw {{item % 2 === 0 ? 'foot-l' : 'foot-r'}}">
+        <view class="bean bean-1"></view><view class="bean bean-2"></view>
+        <view class="bean bean-3"></view><view class="bean bean-4"></view>
+        <view class="pad"></view>
+      </view>
     </view>
   </view>
   <view class="boot-name">肉垫计划</view>