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/ChatCardOther/index.vue

@@ -1,5 +1,5 @@
 <template>
-  <view class="w-full border-box flex flex-col overflow-hidden pl-[12px] mt-6 mb-6">
+  <view class="w-full box-border flex flex-col overflow-hidden pl-[12px] mt-6 mb-6">
     <text class="font-size-14 color-333">{{ text }}</text>
     <slot></slot>
   </view>

src/pages/ChatMain/ChatLongAnswer/index.vue

@@ -9,7 +9,7 @@
     <scroll-view class="flex-full overflow-hidden chat-scroll" scroll-y :scroll-into-view="scrollIntoViewId"
       scroll-with-animation @scroll="onScroll" @touchstart="onTouchStart" @touchend="onTouchEnd"
       @touchcancel="onTouchEnd">
-      <view class="flex flex-col pt-[12px] px-[16px] pb-[24px] border-box gap-10">
+      <view class="flex flex-col pt-[12px] px-[16px] pb-[24px] box-border gap-10">
         <template v-if="hasStructuredLongTextData">
           <view v-if="headerSections.title || headerSections.tag" class="long-answer-header">
             <view v-if="headerSections.title" class="long-answer-title">

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>

src/pages/ChatMain/ChatQuickAccess/index.vue

@@ -1,7 +1,7 @@
 <template>
   <view>
     <view class="quick-access flex flex-row ml-[12px] pt-[8px] pb-[8px] scroll-x whitespace-nowrap">
-      <view class="item border-box rounded-50 flex flex-row items-center" v-for="(item, index) in itemList" :key="index"
+      <view class="item box-border rounded-50 flex flex-row items-center" v-for="(item, index) in itemList" :key="index"
         @click="sendReply(item)">
         <view class="flex items-center justify-center">
           <image v-if="item.icon" class="icon" :src="item.icon" />

src/pages/ChatMain/ChatTopWelcome/index.vue

@@ -1,8 +1,8 @@
 v
 <template>
-  <view class="welcome-content border-box p-[12px]">
+  <view class="welcome-content box-border p-[12px]">
     <view class="wrap rounded-20">
-      <view class="flex flex-items-center flex-justify-between border-box pl-[12px] pr-[12px]">
+      <view class="flex flex-items-center flex-justify-between box-border pl-[12px] pr-[12px]">
         <SpriteAnimator :src="spriteStyle.ipLargeImage" :frameWidth="spriteStyle.frameWidth"
           :frameHeight="spriteStyle.frameHeight" :totalFrames="spriteStyle.totalFrames" :columns="spriteStyle.columns"
           :displayWidth="spriteStyle.displayWidth" :fps="16" />

src/pages/ChatMain/NoticeMessage/detail.vue

@@ -7,7 +7,7 @@
 
     <!-- ✅ 滚动区域 -->
     <scroll-view class="flex-full overflow-hidden" scroll-y>
-      <view class="pb-[24px] border-box overflow-hidden">
+      <view class="pb-[24px] box-border overflow-hidden">
         <image v-if="coverImage" class="w-full h-auto block" :src="coverImage" mode="widthFix" />
 
         <view class="pt-[16px] px-[12px] pb-[24px] border-box">