fix: pairing required(metadata-upgrade) + agents 骨架屏

pairing.rs:
- platform 改用 std::env::consts::OS (windows/macos/linux)，
  新增 deviceFamily=desktop，与 device.rs connect frame 保持一致
- 已配对设备若 platform 字段不匹配，自动覆盖更新，避免 Gateway
  因 metadata-upgrade 拒绝静默自动配对

agents.js + components.css:
- loadAgents 调用前先渲染 3 条骨架屏占位，消除空白等待期
- 添加 .skeleton 闪光动画样式

src/pages/agents.js

@@ -34,8 +34,23 @@ export async function render() {
   return page
 }
 
+function renderSkeleton(container) {
+  const item = () => `
+    <div class="agent-card" style="pointer-events:none">
+      <div class="agent-card-header">
+        <div class="skeleton" style="width:40px;height:40px;border-radius:50%"></div>
+        <div style="flex:1;display:flex;flex-direction:column;gap:6px">
+          <div class="skeleton" style="width:45%;height:16px;border-radius:4px"></div>
+          <div class="skeleton" style="width:60%;height:12px;border-radius:4px"></div>
+        </div>
+      </div>
+    </div>`
+  container.innerHTML = [item(), item(), item()].join('')
+}
+
 async function loadAgents(page, state) {
   const container = page.querySelector('#agents-list')
+  renderSkeleton(container)
   try {
     state.agents = await api.listAgents()
     renderAgents(page, state)

src/style/components.css

@@ -1,3 +1,15 @@
+/* 骨架屏 */
+.skeleton {
+  background: linear-gradient(90deg, var(--bg-secondary) 25%, var(--bg-tertiary, var(--bg-card-hover)) 50%, var(--bg-secondary) 75%);
+  background-size: 200% 100%;
+  animation: skeleton-shine 1.4s ease infinite;
+}
+
+@keyframes skeleton-shine {
+  0% { background-position: 200% 0; }
+  100% { background-position: -200% 0; }
+}
+
 /* 卡片 */
 .card {
   background: var(--bg-card);