style: migrate font-size classes to text-[size] syntax

Additionally update spacing utilities and refactor the OrderQrcode component:
- Switch popup implementation from uni-popup to van-popup
- Remove the deprecated SCSS stylesheet
- Clean up component template and script code

src/components/Stepper/index.vue

@@ -1,9 +1,7 @@
 <template>
   <div class="stepper-wrapper border-box flex flex-items-center rounded-8">
     <uni-icons type="minus" size="24" color="#D1D1D1" @click="decrease" />
-    <text
-      class="stepper-text text-center font-size-14 font-500 color-000 ml-4 mr-4"
-    >
+    <text class="stepper-text text-center text-[14px] font-500 color-000 ml-4 mr-4">
       {{ value }} {{ unit }}
     </text>
     <uni-icons type="plus" size="24" color="#198CFF" @click="increase" />