fix: unify input field widths and fix button spacing in boxing page

- Unify all input fields (box number and quantity) to width 52 in both
  single-code and multi-code boxing modes
- Flatten nested Row layout in multi-code submit row to ensure equal
  8px spacing between quantity input, confirm button, and complete-box
  button

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

lib/pages/boxing_page.dart

@@ -2164,7 +2164,7 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
             ),
             const SizedBox(width: 4),
             SizedBox(
-              width: 70,
+              width: 52,
               height: 34,
               child: TextField(
                 controller: _quantityController,
@@ -2762,12 +2762,6 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
   Widget _buildManyToOneSubmitRow(bool hasScan, bool hasItems) {
     return Padding(
       padding: const EdgeInsets.fromLTRB(12, 7, 12, 8),
-      child: Row(
-        crossAxisAlignment: CrossAxisAlignment.center,
-        children: [
-          // Left half: quantity label + input + confirm
-          Expanded(
-            flex: 1,
       child: Row(
         crossAxisAlignment: CrossAxisAlignment.center,
         children: [
@@ -2780,8 +2774,8 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
             ),
           ),
           const SizedBox(width: 6),
-                Expanded(
-                  child: SizedBox(
+          SizedBox(
+            width: 52,
             height: 34,
             child: TextField(
               controller: _quantityController,
@@ -2789,11 +2783,12 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
               enabled: hasScan && !_isSubmitting,
               keyboardType: TextInputType.none,
               inputFormatters: [FilteringTextInputFormatter.digitsOnly],
+              textAlign: TextAlign.center,
               onChanged: (_) => setState(() {}),
               onEditingComplete: () {},
               onSubmitted: (_) => _submitFromKeyboard(),
               decoration: InputDecoration(
-                        contentPadding: const EdgeInsets.symmetric(horizontal: 10),
+                contentPadding: EdgeInsets.zero,
                 border: OutlineInputBorder(
                   borderRadius: BorderRadius.circular(6),
                 ),
@@ -2818,7 +2813,6 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
               ),
             ),
           ),
-                ),
           const SizedBox(width: 8),
           SizedBox(
             height: 34,
@@ -2854,13 +2848,8 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
                     ),
             ),
           ),
-              ],
-            ),
-          ),
           const SizedBox(width: 8),
-          // Right half: 完成本箱
           Expanded(
-            flex: 1,
             child: SizedBox(
               height: 34,
               child: OutlinedButton(