feat: 数据看板视频封面接口 + Cookie 编辑弹窗全屏化

- Controllers/VideoController.cs：补回 upstream merge 时丢失的 GetVideoCover 接口（/api/video/cover/{id}），修复数据看板作者视频列表封面加载失败
- app/src/App.vue：.full-modal 改成 flex + height:100vh 全屏抽屉式布局，footer 常驻底部，body flex:1 承担滚动，解决大视口下 modal 底部大片空白
- app/src/pages/cok/CookieTable.vue：删除 .modal-scroll-body 的双层滚动 hack（max-height + overflow:scroll），让 .ant-modal-body 独家承担滚动，修复小屏下 Cookie 编辑弹窗内容滑不动

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/pages/cok/CookieTable.vue

@@ -707,26 +707,8 @@ const switchdownCollect = (e: any) => {
 
 <style scoped lang="less">
 .modal-scroll-body {
-  max-height: calc(100vh - 160px);
-  overflow-y: scroll;
-  overflow-x: hidden;
   padding-right: 4px;
 }
-.modal-scroll-body::-webkit-scrollbar {
-  -webkit-appearance: none;
-  width: 8px;
-}
-.modal-scroll-body::-webkit-scrollbar-track {
-  background: #f0f0f0;
-  border-radius: 4px;
-}
-.modal-scroll-body::-webkit-scrollbar-thumb {
-  background: #bbb;
-  border-radius: 4px;
-}
-.modal-scroll-body::-webkit-scrollbar-thumb:hover {
-  background: #888;
-}
 .ant-form-item {
   margin-bottom: 10px;
 }