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/widgets/multi_select_dropdown.dart

@@ -176,31 +176,37 @@ class _FilterableColumnHeaderState extends State<FilterableColumnHeader> {
       onEnter: (_) => setState(() => _hovered = true),
       onExit: (_) => setState(() => _hovered = false),
       child: Row(
-        mainAxisSize: MainAxisSize.min,
+        mainAxisAlignment: MainAxisAlignment.spaceBetween,
         children: [
           Text(widget.text),
-          AnimatedOpacity(
-            opacity: showIcon ? 1.0 : 0.0,
-            duration: const Duration(milliseconds: 120),
-            child: InkWell(
-              onTap: widget.options.isEmpty ? null : () => _show(context),
-              borderRadius: BorderRadius.circular(4),
-              child: Padding(
-                padding: const EdgeInsets.symmetric(horizontal: 2),
-                child: Icon(
-                  active ? Icons.filter_alt : Icons.filter_alt_outlined,
-                  size: 13,
-                  color: active ? AppTheme.primary : AppTheme.textSecondary,
+          Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              AnimatedOpacity(
+                opacity: showIcon ? 1.0 : 0.0,
+                duration: const Duration(milliseconds: 120),
+                child: InkWell(
+                  onTap: widget.options.isEmpty ? null : () => _show(context),
+                  borderRadius: BorderRadius.circular(4),
+                  child: Padding(
+                    padding: const EdgeInsets.symmetric(horizontal: 3),
+                    child: Icon(
+                      active ? Icons.filter_alt : Icons.filter_alt_outlined,
+                      size: 16,
+                      color: active ? AppTheme.primary : AppTheme.textSecondary,
+                    ),
+                  ),
                 ),
               ),
-            ),
+              if (active)
+                InkWell(
+                  onTap: () => widget.onChanged({}),
+                  borderRadius: BorderRadius.circular(4),
+                  child: const Icon(Icons.cancel, size: 14,
+                      color: AppTheme.primary),
+                ),
+            ],
           ),
-          if (active)
-            InkWell(
-              onTap: () => widget.onChanged({}),
-              borderRadius: BorderRadius.circular(4),
-              child: const Icon(Icons.cancel, size: 12, color: AppTheme.primary),
-            ),
         ],
       ),
     );