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/FeedHeader.tsx

@@ -1,3 +1,4 @@
+import { Users, FileText, LayoutGrid } from 'lucide-react';
 import { useNavigate } from 'react-router-dom';
 import type { Board, ForumStats } from '../api/types';
 
@@ -24,13 +25,20 @@ export default function FeedHeader({ boardId, keyword, boards, stats, postTotal
       <div className="feed-head__title">
         <h2 title={boardHint || undefined}>{title}</h2>
         {!keyword && stats && (
-          <span className="feed-head__meta">
-            会员 <strong>{stats.users}</strong>
-            <span className="feed-head__dot" aria-hidden>·</span>
-            帖子 <strong>{stats.posts}</strong>
-            <span className="feed-head__dot" aria-hidden>·</span>
-            板块 <strong>{stats.boards}</strong>
-          </span>
+          <div className="feed-head__stats">
+            <span className="feed-stat-chip">
+              <Users aria-hidden />
+              会员 <strong>{stats.users}</strong>
+            </span>
+            <span className="feed-stat-chip">
+              <FileText aria-hidden />
+              帖子 <strong>{stats.posts}</strong>
+            </span>
+            <span className="feed-stat-chip">
+              <LayoutGrid aria-hidden />
+              板块 <strong>{stats.boards}</strong>
+            </span>
+          </div>
         )}
       </div>
       {keyword && (