feat(admin): 刷新改为独立的圆形图标按钮（42px），主操作按钮单独一行

- 12 个后台页的刷新统一为 .refresh-btn，固定在页头最右，悬停旋转
- 渠道管理「添加渠道」、并发分组「新增分组」移到页头下方单独一行
- 渠道空状态文案同步

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frontend/src/views/ConcurrencyView.vue

@@ -57,11 +57,12 @@ onMounted(load)
         <p class="page-sub">每个分组限制成员用户的<strong>同时生成数</strong>(画图台 + API key 合计)。<strong>0 = 不限制</strong>。新用户自动进入「默认注册分组」。</p>
       </div>
       <div class="page-actions">
-        <button @click="load" class="btn-soft"><Icon name="refresh" class="w-3.5 h-3.5" /> 刷新</button>
-        <button @click="openNew" class="btn-primary shrink-0">+ 新增分组</button>
+        <button @click="load" class="refresh-btn" title="刷新" aria-label="刷新"><Icon name="refresh" :size="18" /></button>
       </div>
     </div>
 
+    <div class="page-bar"><button @click="openNew" class="btn-primary shrink-0"><Icon name="plus" class="w-4 h-4" /> 新增分组</button></div>
+
     <div class="card overflow-hidden">
       <table class="w-full text-sm">
         <thead>