feat: SSE Last-Event-ID 断线重放 + GUI 表单防刷新

- webui handler: 新增 sseEventRing 环状缓冲区(200条)，断线重连按 Last-Event-ID 重放遗漏事件
- GUI app.js: doRenderAll 检测连接表单打开时改走 refreshDataOnly，修复 15s 定时器擦掉用户输入的 bug
- 附带 dashboard.html/index.html 前端调整 + handler_sse_test.go 单测

cmd/gui/renderer/index.html

@@ -18,6 +18,10 @@
       src="https://cdnjs.cloudflare.com/ajax/libs/marked/4.3.0/marked.min.js"
       onerror="console.warn('marked CDN failed')"
     ></script>
+    <script
+      src="https://cdn.jsdelivr.net/npm/dompurify@3.2.4/dist/purify.min.js"
+      onerror="console.warn('DOMPurify CDN failed')"
+    ></script>
     <script>
       setTimeout(function () {
         if (!window.THREE) window._THREE_FAILED = true;