feat: 首页配置封面图上传预览，修复系列活动跳转，活动卡片样式优化

backend/competition/admin.py

@@ -7,12 +7,13 @@ from .models import Competition, CompetitionEnrollment, ScoreDimension, Project,
 
 @admin.register(HomePageConfig)
 class HomePageConfigAdmin(ModelAdmin):
-    list_display = ['id', 'main_title', 'organizer', 'undertaker', 'is_active']
+    list_display = ['id', 'main_title', 'organizer', 'undertaker', 'is_active', 'banner_preview']
     list_editable = ['main_title', 'organizer', 'undertaker', 'is_active']
+    readonly_fields = ['banner_preview']
     fieldsets = (
         ('封面图', {
-            'fields': ('banner_image', 'banner_image_url'),
-            'description': '首页标题下方的封面图，可上传本地图片或填写URL'
+            'fields': ('banner_image', 'banner_image_url', 'banner_preview'),
+            'description': '首页标题下方的封面图，可上传本地图片或填写URL，优先使用上传的图片'
         }),
         ('标题设置', {
             'fields': ('main_title',)
@@ -25,6 +26,18 @@ class HomePageConfigAdmin(ModelAdmin):
         }),
     )
 
+    @display(description="封面图预览")
+    def banner_preview(self, obj):
+        url = None
+        if obj.banner_image and obj.banner_image.name:
+            from django.conf import settings
+            url = settings.MEDIA_URL + obj.banner_image.name
+        elif obj.banner_image_url:
+            url = obj.banner_image_url
+        if url:
+            return format_html('<img src="{}" style="max-height:120px;max-width:300px;border-radius:6px;object-fit:cover;" />', url)
+        return "暂无图片"
+
 
 class ScoreDimensionInline(admin.TabularInline):
     model = ScoreDimension