refactor(CheckinMap): 优化签到卡片布局与样式

重构卡片模板结构，将日期栏与进入按钮分组为底部模块；修复封面图片显示问题；统一使用flex布局优化整体排版

@@ -18,12 +18,14 @@
         <!-- 卡片内容 -->
         <view class="card-content">
           <text class="card-title">{{ item.title }}</text>
-          <view class="card-time-wrapper">
-            <text class="card-time-label">活动日期</text>
-            <text class="card-time">{{ item.timeRange }}</text>
-          </view>
-          <view class="enter-btn" @tap.stop="handleEnter(item)">
-            <text class="enter-btn-text">立即进入</text>
+          <view class="card-bottom-module">
+            <view class="card-time-wrapper">
+              <text class="card-time-label">活动日期</text>
+              <text class="card-time">{{ item.timeRange }}</text>
+            </view>
+            <view class="enter-btn" @tap.stop="handleEnter(item)">
+              <text class="enter-btn-text">立即进入</text>
+            </view>
           </view>
         </view>
       </view>
@@ -178,6 +180,8 @@ useLoad(() => {
 }
 
 .map-card {
+  display: flex;
+  flex-direction: column;
   background-color: #ffffff;
   border-radius: 16px;
   overflow: hidden;
@@ -197,7 +201,9 @@ useLoad(() => {
 
 .cover-image {
   width: 100%;
-  /* 高度由 mode="widthFix" 自动控制，保持宽高比 */
+  height: 100%;
+  display: block;
+  object-fit: cover;
 }
 
 .card-content {
@@ -205,6 +211,7 @@ useLoad(() => {
   display: flex;
   flex-direction: column;
   gap: 12px;
+  flex: 1;
 }
 
 .card-title {
@@ -218,6 +225,13 @@ useLoad(() => {
   -webkit-box-orient: vertical;
 }
 
+.card-bottom-module {
+  margin-top: auto;
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+}
+
 .card-time-wrapper {
   display: flex;
   flex-direction: column;