支持板块图标与主题色自定义，并优化列表加载与未保存离开提示。

新增后端图标校验与色槽规范化，前端展示 BoardBadge/骨架屏，发帖与板块管理页增加未保存确认。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/PostListSkeleton.tsx

@@ -0,0 +1,30 @@
+import { Skeleton } from '@/components/ui/skeleton';
+
+interface Props {
+  count?: number;
+}
+
+/** 帖子列表加载骨架屏 */
+export default function PostListSkeleton({ count = 8 }: Props) {
+  return (
+    <div className="post-list-skeleton" aria-busy="true" aria-label="加载中">
+      {Array.from({ length: count }, (_, i) => (
+        <div key={i} className="post-row post-row--skeleton">
+          <Skeleton className="skeleton--avatar" />
+          <div className="post-body">
+            <Skeleton className="skeleton--title" style={{ width: `${55 + (i % 4) * 10}%` }} />
+            <div className="skeleton-meta-row">
+              <Skeleton className="skeleton--badge" />
+              <Skeleton className="skeleton--meta" />
+              <Skeleton className="skeleton--meta skeleton--meta-short" />
+            </div>
+          </div>
+          <div className="post-stats">
+            <Skeleton className="skeleton--stat" />
+            <Skeleton className="skeleton--stat" />
+          </div>
+        </div>
+      ))}
+    </div>
+  );
+}