feat(client): 所有表格列头加筛选，图标更大靠右对齐

FilterableColumnHeader：
- mainAxisAlignment.spaceBetween 将图标推到列头右侧
- 漏斗图标从 13px 增大到 16px，清除图标 14px

各页面改动：
- batch_tracking：状态/仓库/供应商筛选移入列头，工具栏合并为一行
- finance：类型/往来单位筛选移入列头，工具栏保留月份选择和显示字段
- inventory：仓库筛选从 API 端单选改为客户端多选，移入"仓库"列头
- products：品牌列头加筛选，从加载数据中动态派生选项

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/screens/products/products_screen.dart

@@ -5,6 +5,7 @@ import '../../core/theme/app_theme.dart';
 import '../../models/product.dart';
 import '../../providers/product_provider.dart';
 import '../../widgets/data_table_card.dart';
+import '../../widgets/multi_select_dropdown.dart' show FilterableColumnHeader;
 import '../../widgets/page_scaffold.dart';
 
 class ProductsScreen extends ConsumerStatefulWidget {
@@ -17,6 +18,7 @@ class ProductsScreen extends ConsumerStatefulWidget {
 class _ProductsScreenState extends ConsumerState<ProductsScreen> {
   final _searchCtrl = TextEditingController();
   Timer? _debounce;
+  Set<String> _filterBrand = {};
 
   @override
   void dispose() {
@@ -133,6 +135,17 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
 
   Widget _buildProductList(
       List<Product> products, int totalCount, int page) {
+    // 品牌选项从当前数据中派生，客户端筛选
+    final brandOptions = products
+        .map((p) => p.brand ?? '')
+        .where((s) => s.isNotEmpty)
+        .toSet()
+        .toList()
+      ..sort();
+    final filteredProducts = _filterBrand.isEmpty
+        ? products
+        : products.where((p) => _filterBrand.contains(p.brand ?? '')).toList();
+
     return DataTableCard(
       totalCount: totalCount,
       page: page,
@@ -160,18 +173,25 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
           ),
         ],
       ),
-      columns: const [
-        DataColumn(label: Text('商品编码')),
-        DataColumn(label: Text('商品名称')),
-        DataColumn(label: Text('品牌')),
-        DataColumn(label: Text('规格')),
-        DataColumn(label: Text('单位')),
-        DataColumn(label: Text('进价'), numeric: true),
-        DataColumn(label: Text('售价'), numeric: true),
-        DataColumn(label: Text('安全库存'), numeric: true),
-        DataColumn(label: Text('操作')),
+      columns: [
+        const DataColumn(label: Text('商品编码')),
+        const DataColumn(label: Text('商品名称')),
+        DataColumn(
+          label: FilterableColumnHeader(
+            text: '品牌',
+            options: brandOptions,
+            selected: _filterBrand,
+            onChanged: (v) => setState(() => _filterBrand = v),
+          ),
+        ),
+        const DataColumn(label: Text('规格')),
+        const DataColumn(label: Text('单位')),
+        const DataColumn(label: Text('进价'), numeric: true),
+        const DataColumn(label: Text('售价'), numeric: true),
+        const DataColumn(label: Text('安全库存'), numeric: true),
+        const DataColumn(label: Text('操作')),
       ],
-      rows: products.isEmpty
+      rows: filteredProducts.isEmpty
           ? [
               const DataRow(cells: [
                 DataCell(SizedBox()),
@@ -186,7 +206,7 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
                 DataCell(SizedBox()),
               ])
             ]
-          : products
+          : filteredProducts
               .map((p) => DataRow(
                     cells: [
                       DataCell(Text(p.code,