refactor: replace border-box with box-border and clean up unused scss

Remove the unused box-sizing.scss stylesheet and its import from the main SCSS index file.
Update all Vue component template class references from the custom border-box class to the
native box-border utility class to eliminate redundant custom CSS and align with the project's
utility class conventions.

src/pages/ChatMain/ChatMoreTips/index.vue

@@ -1,15 +1,8 @@
 <template>
   <view class="more-tips bg-white border-box">
     <view class="font-size-0 whitespace-nowrap scroll-x">
-      <view
-        class="more-tips-item border-box inline-block mr-8"
-        v-for="(item, index) in guideWords"
-        :key="index"
-      >
-        <text
-          :class="['font-500 font-size-12 text-center', `color-${index}`]"
-          @click="sendReply(item)"
-        >
+      <view class="more-tips-item box-border inline-block mr-8" v-for="(item, index) in guideWords" :key="index">
+        <text :class="['font-500 font-size-12 text-center', `color-${index}`]" @click="sendReply(item)">
           {{ item }}
         </text>
       </view>