优化前端样式：统一同一行按钮水平对齐与垂直居中，清理多余临时文件

frontend/src/App.css

@@ -42,4 +42,9 @@ body {
 button {
   font: inherit;
   cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  vertical-align: middle;
+  text-align: center;
 }

frontend/src/views/DNSAccounts/DNSAccounts.css

@@ -136,6 +136,7 @@ select:focus-visible,
 
 .dns-row-actions {
   display: flex;
+  align-items: center;
   flex-wrap: wrap;
   gap: 6px;
 }

frontend/src/views/Dashboard/SettingsModal.css

@@ -21,6 +21,7 @@
 
 .settings-tabs {
   display: flex;
+  align-items: center;
   flex: 0 0 auto;
   gap: 6px;
   padding-bottom: 12px;

frontend/src/views/UserManagement/UserManagement.css

@@ -297,6 +297,7 @@ button:focus-visible {
 }
 .modal-actions {
   display: flex;
+  align-items: center;
   justify-content: flex-end;
   flex-wrap: wrap;
   gap: 10px;