fix(symbol-manager): Select.Item empty string replaced with '__none__' placeholder

src/components/admin/symbol-manager.tsx

@@ -719,14 +719,14 @@ function SymbolCard({
 
       {/* Category select */}
       <Select
-        value={sym.categoryId || ''}
-        onValueChange={onMoveCategory}
+        value={sym.categoryId || '__none__'}
+        onValueChange={(val) => onMoveCategory(val === '__none__' ? '' : val)}
       >
         <SelectTrigger className="h-5 text-[10px] border-0 bg-transparent px-0 mt-0.5 hover:bg-muted/50 rounded">
           <SelectValue placeholder="Kategorie" />
         </SelectTrigger>
         <SelectContent>
-          <SelectItem value="">Ohne Kategorie</SelectItem>
+          <SelectItem value="__none__">Ohne Kategorie</SelectItem>
           {categories.map(c => (
             <SelectItem key={c.id} value={c.id}>{c.name}</SelectItem>
           ))}