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

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/views/OverviewView.vue

@@ -155,7 +155,7 @@ onUnmounted(() => clearInterval(timer))
         <span class="pill" :class="'pill--' + overallHealth.tone">
           <span class="dot" :class="'dot--' + overallHealth.tone"></span>{{ overallHealth.label }}
         </span>
-        <button @click="refreshAll" class="ov-btn"><Icon name="refresh" :size="14" /> 刷新</button>
+        <button @click="refreshAll" class="refresh-btn" title="刷新" aria-label="刷新"><Icon name="refresh" :size="18" /></button>
       </div>
     </div>