fix(knowledge): enforce upload type and size limits on client

Why: 添加知识库弹窗只在 hint 文案里写了「docx/pdf/txt/md/xls/xlsx，<=30M」，
beforeUpload 没做校验，用户可以提交任意大小、任意格式的文件后再被后端拒绝。
现在前端按白名单扩展名 + 30MB 阈值拦截，并给 dragger 加 accept 让系统选择器
默认过滤非法格式。

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

apps/admin-web/src/views/KnowledgeView.vue

@@ -21,6 +21,10 @@ type KnowledgeSourceType = 'document' | 'website' | 'text'
 type KnowledgeGroupLevel = 'root' | 'child'
 const MAX_WEBSITE_URLS = 3
 const MAX_TEXT_NAME_LENGTH = 20
+const MAX_UPLOAD_SIZE_MB = 30
+const MAX_UPLOAD_SIZE_BYTES = MAX_UPLOAD_SIZE_MB * 1024 * 1024
+const ALLOWED_UPLOAD_EXTENSIONS = ['docx', 'pdf', 'txt', 'md', 'xls', 'xlsx'] as const
+const UPLOAD_ACCEPT_ATTR = ALLOWED_UPLOAD_EXTENSIONS.map((ext) => `.${ext}`).join(',')
 
 interface TreeNode {
   title: string
@@ -458,7 +462,19 @@ function handleTreeSelect(keys: (string | number)[]): void {
 }
 
 const beforeUpload: UploadProps['beforeUpload'] = (file) => {
-  uploadFiles.value.push(file as File)
+  const rawFile = file as File
+  const ext = rawFile.name.includes('.')
+    ? rawFile.name.slice(rawFile.name.lastIndexOf('.') + 1).toLowerCase()
+    : ''
+  if (!ALLOWED_UPLOAD_EXTENSIONS.includes(ext as (typeof ALLOWED_UPLOAD_EXTENSIONS)[number])) {
+    message.error(`「${rawFile.name}」格式不支持，仅支持 ${ALLOWED_UPLOAD_EXTENSIONS.join('、')}`)
+    return false
+  }
+  if (rawFile.size > MAX_UPLOAD_SIZE_BYTES) {
+    message.error(`「${rawFile.name}」超过 ${MAX_UPLOAD_SIZE_MB}MB 大小限制`)
+    return false
+  }
+  uploadFiles.value.push(rawFile)
   return false
 }
 
@@ -718,6 +734,7 @@ function closeItemDetail(): void {
             :before-upload="beforeUpload"
             :file-list="uploadFiles"
             :multiple="true"
+            :accept="UPLOAD_ACCEPT_ATTR"
             class="custom-dragger"
             @remove="handleRemoveFile"
           >