refactor(form): 优化表单横幅图片的加载逻辑

将硬编码的横幅图片路径改为动态读取配置项，无配置时使用默认图片

pages_tool/form/formdata.vue

@@ -3,7 +3,7 @@
 		<block v-if="detail">
 			<view class="page">
 				<view class="form-banner" v-if="detail.show_banner > 0">
-					<image :src="$util.img('public/uniapp/form/banner.png')" mode="widthFix"></image>
+					<image :src="$util.img(formBanner)" mode="widthFix"></image>
 				</view>
 				<view class="form-tip">
 					<view class="system-form-tip" v-if="detail.show_tip > 0">{{ formTip }}</view>
@@ -42,6 +42,10 @@ export default {
 		}
 	},
 	computed: {
+		formBanner() {
+			const bannerImg = this.detail?.form_banner
+			return bannerImg ? bannerImg : 'public/uniapp/form/banner.png'
+		},
 		formTip() {
 			const tip = this.detail?.form_tip
 			console.log('formTip computed, tip value:', tip, 'truthy:', !!tip)