fix(data-ui): 导入类别改为常驻复选框，取消勾选不再从列表消失

之前顶部类别取自服务端返回的「本次已选类别」，取消一个后列表被重算，那一项直接消失 ——
把勾选框做成了单向筛选。现在改为取包头里的全部类别（preview.file.categories）常驻显示，
勾选状态只决定预览与导入范围；取消最后一类时给明确提示，不再静默无响应。

frontend/src/pages/DataTransfer.jsx

@@ -71,6 +71,7 @@ function DataTransfer() {
   const [result, setResult] = useState(null)
   const [percent, setPercent] = useState(0)
   const [replanning, setReplanning] = useState(false)
+  const [categoryHint, setCategoryHint] = useState('')
   const [dragging, setDragging] = useState(false)
   const inputRef = useRef(null)
 
@@ -150,6 +151,7 @@ function DataTransfer() {
     setPreview(null)
     setResult(null)
     setPercent(0)
+    setCategoryHint('')
     setPhase('checking')
     setError('')
 
@@ -175,11 +177,16 @@ function DataTransfer() {
 
   /** 切换类别后重算预览：包已经在服务器上，不用重传 */
   const toggleImport = async key => {
-    if (replanning) return
-    const next = importPicks.includes(key)
-      ? importPicks.filter(item => item !== key)
-      : [...importPicks, key]
-    if (!next.length) return
+    if (replanning || phase === 'applying') return
+    const on = importPicks.includes(key)
+    /* 全部取消等于「什么都不导」，服务端把空列表当成「全选」——
+       所以这里明确挡住，并说明原因，而不是点了没反应 */
+    if (on && importPicks.length === 1) {
+      setCategoryHint('至少要勾一类。想放弃这次导入，点下面的「重新选择」。')
+      return
+    }
+    setCategoryHint('')
+    const next = on ? importPicks.filter(item => item !== key) : [...importPicks, key]
     setImportPicks(next)
     setReplanning(true)
     setError('')
@@ -228,6 +235,7 @@ function DataTransfer() {
     setResult(null)
     setImportPicks([])
     setPercent(0)
+    setCategoryHint('')
     setError('')
     if (inputRef.current) inputRef.current.value = ''
   }
@@ -241,7 +249,10 @@ function DataTransfer() {
 
   const busy = phase === 'checking' || phase === 'applying' || replanning
   const plan = preview?.plan
-  const availableKeys = preview?.availableCategories || preview?.file?.categories || []
+  /* 顶部类别必须取「包里有哪几类」，而不是「这次选了哪几类」——
+     后者会随着取消勾选而缩短，表现就是「点一下那一类就消失了」。
+     preview.file.categories 来自包头，重算预览也不会变。 */
+  const categoriesInFile = preview?.file?.categories || []
 
   return (
     <div className="page-container transfer-page">
@@ -380,24 +391,25 @@ function DataTransfer() {
 
               <div className="transfer-cats">
                 <span className="muted-note">导入类别</span>
-                {availableKeys.map(key => {
+                {categoriesInFile.map(key => {
                   const category = categoryByKey.get(key)
                   const on = importPicks.includes(key)
                   return (
-                    <button
-                      key={key}
-                      type="button"
-                      className={`chip-filter${on ? ' is-active' : ''}`}
-                      onClick={() => toggleImport(key)}
-                      disabled={replanning || phase === 'applying'}
-                      aria-pressed={on}
-                    >
+                    <label key={key} className={`chip-filter transfer-cat${on ? ' is-active' : ''}`}>
+                      <input
+                        type="checkbox"
+                        className="transfer-cat-box"
+                        checked={on}
+                        disabled={replanning || phase === 'applying'}
+                        onChange={() => toggleImport(key)}
+                      />
                       {category?.label || key}
-                    </button>
+                    </label>
                   )
                 })}
                 {replanning && <span className="spinner" />}
               </div>
+              {categoryHint && <p className="help-text transfer-hint">{categoryHint}</p>}
 
               <div className="table-wrap transfer-report">
                 <table className="data-table">