fix(mail): 修复邮件列表工具栏标题与选中计数换行错乱

- 标题与「已选 x 封」计数包裹进同一 flex 容器（align-items: baseline） - white-space nowrap 禁止换行、计数 flex-shrink 0、工具栏 overflow hidden

src/views/mail/Mailbox.vue

@@ -602,8 +602,10 @@ function isSelected(id: string): boolean {
           <section class="mail-list-pane">
             <div class="list-toolbar">
               <el-checkbox :model-value="allChecked" :indeterminate="someChecked" @change="toggleSelectAll" title="全选" />
-              <span class="toolbar-title">{{ currentFolderName }}</span>
-              <span v-if="selectedIds.length" class="toolbar-count">已选 {{ selectedIds.length }} 封</span>
+              <div class="toolbar-left">
+                <span class="toolbar-title">{{ currentFolderName }}</span>
+                <span v-if="selectedIds.length" class="toolbar-count">已选 {{ selectedIds.length }} 封</span>
+              </div>
               <div class="spacer" />
               <el-tooltip content="刷新">
                 <el-button text size="small" :icon="Refresh" @click="loadEmails" />
@@ -960,18 +962,29 @@ html.dark .mail-login-wrap {
   gap: 6px;
   padding: 0 10px;
   border-bottom: 1px solid var(--el-border-color-lighter);
+  overflow: hidden;
+  white-space: nowrap;
 }
 .list-toolbar .spacer {
   flex: 1;
 }
+.toolbar-left {
+  display: flex;
+  align-items: baseline;
+  gap: 8px;
+  min-width: 0;
+}
 .toolbar-title {
   font-size: 14px;
   font-weight: 600;
   color: var(--el-text-color-primary);
+  white-space: nowrap;
 }
 .toolbar-count {
   font-size: 12px;
   color: var(--el-color-primary);
+  white-space: nowrap;
+  flex-shrink: 0;
 }
 .email-list {
   flex: 1;