fix: clarify signature reporting status

src/apps/admin/AdminDrainageFieldsPage.tsx

@@ -34,6 +34,7 @@ export function AdminDrainageFieldsPage() {
   const [fieldType, setFieldType] = useState<ReportFieldType>('string');
   const [description, setDescription] = useState('');
   const [error, setError] = useState('');
+  const codeError = code && !/^[A-Za-z0-9]+$/.test(code) ? '字段代码只能包含阿拉伯数字和英文大小写字母' : '';
 
   function loadData() {
     adminApi.listDrainageFields()
@@ -106,7 +107,7 @@ export function AdminDrainageFieldsPage() {
         footer={(
           <>
             <Button onClick={() => setCreating(false)} variant="ghost">取消</Button>
-            <Button disabled={!code || !name} onClick={createField}>保存</Button>
+            <Button disabled={!code || !name || Boolean(codeError)} onClick={createField}>保存</Button>
           </>
         )}
         onClose={() => setCreating(false)}
@@ -114,7 +115,7 @@ export function AdminDrainageFieldsPage() {
         title="添加报备字段"
       >
         <div className="admin-system-modal-form">
-          <Input label="字段代码" onChange={(event) => setCode(event.target.value)} value={code} />
+          <Input error={codeError} label="字段代码" onChange={(event) => setCode(event.target.value)} placeholder="仅允许数字和英文字母" value={code} />
           <Input label="字段名称" onChange={(event) => setName(event.target.value)} value={name} />
           <Select
             label="字段类型"