refactor: convert legacy color classes to tailwind syntax

Replace all custom `bg-*` utility classes across Vue components with Tailwind's arbitrary hex color notation (e.g. `bg-[#f7f7f7]`). Remove redundant hardcoded background color definitions from src/static/scss/background.scss, clean up template formatting, and update gradient button styles to use direct hex values.

src/components/SurveyQuestionnaire/index.vue

@@ -10,7 +10,8 @@
 
       <image class="w-full" :src="surveyData.bannerUrl" mode="widthFix" />
 
-      <view class="h-44 m-12 rounded-50 bg-button color-white flex flex-items-center flex-justify-center"
+      <view
+        class="h-44 m-12 rounded-50 bg-gradient-to-r from-[#0CCD58] to-[#4de4ff] color-white flex flex-items-center flex-justify-center"
         @click="handleCall">
         前往填写
       </view>