fix(mobile-app): fix layout constraint error in pending actions page

Wrap ElevatedButton in SizedBox(width: 72) to prevent
BoxConstraints infinite width error in Row layout.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/pending_actions/presentation/pages/pending_actions_page.dart

@@ -493,12 +493,14 @@ class _PendingActionsPageState extends ConsumerState<PendingActionsPage> {
             ),
             // 操作按钮
             if (isCurrent)
-              ElevatedButton(
+              SizedBox(
+                width: 72,
+                child: ElevatedButton(
                   onPressed: _isExecuting ? null : _executeCurrentAction,
                   style: ElevatedButton.styleFrom(
                     backgroundColor: const Color(0xFFD4AF37),
                     foregroundColor: Colors.white,
-                  padding: const EdgeInsets.symmetric(horizontal: 20, vertical: 10),
+                    padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 10),
                     shape: RoundedRectangleBorder(
                       borderRadius: BorderRadius.circular(8),
                     ),
@@ -514,6 +516,7 @@ class _PendingActionsPageState extends ConsumerState<PendingActionsPage> {
                         )
                       : const Text('开始'),
                 ),
+              ),
           ],
         ),
       ),