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/UnsavedChangesDialog.tsx

@@ -0,0 +1,45 @@
+import { Button } from '@/components/ui/button';
+import {
+  AlertDialog,
+  AlertDialogCancel,
+  AlertDialogContent,
+  AlertDialogDescription,
+  AlertDialogFooter,
+  AlertDialogHeader,
+  AlertDialogTitle,
+} from '@/components/ui/alert-dialog';
+
+interface Props {
+  open: boolean;
+  onStay: () => void;
+  onLeave: () => void;
+  /** 编辑已有帖子时为 true */
+  isEdit?: boolean;
+}
+
+export default function UnsavedChangesDialog({ open, onStay, onLeave, isEdit = false }: Props) {
+  return (
+    <AlertDialog open={open}>
+      <AlertDialogContent onEscapeKeyDown={onStay}>
+        <AlertDialogHeader>
+          <AlertDialogTitle>放弃未保存的修改？</AlertDialogTitle>
+          <AlertDialogDescription>
+            {isEdit
+              ? '你对这篇文章的修改尚未保存，离开后将无法恢复。'
+              : '你正在撰写的内容尚未发布，离开后将无法恢复。'}
+          </AlertDialogDescription>
+        </AlertDialogHeader>
+        <AlertDialogFooter>
+          <AlertDialogCancel onClick={onStay}>继续编辑</AlertDialogCancel>
+          <Button
+            type="button"
+            variant="destructive"
+            onClick={onLeave}
+          >
+            放弃并离开
+          </Button>
+        </AlertDialogFooter>
+      </AlertDialogContent>
+    </AlertDialog>
+  );
+}