feat(widget): Widget 配置、loader 与右下角悬浮入口

生成可嵌入站点的 loader 与悬浮按钮，支持展开收起和移动端；附独立宿主测试页。

frontend/public/meta-oauth-callback.html

@@ -0,0 +1,29 @@
+<!doctype html>
+<!--
+  OAuth redirect target for Facebook Login for Business (Messenger/Instagram).
+
+  Meta redirects the login popup here with ?code&state; this relays them to the
+  opener and closes. It's a static file (not the SPA) so the popup stays light,
+  and it must be registered as a Valid OAuth Redirect URI on the Meta app.
+-->
+<html>
+  <head><meta charset="utf-8" /><title>Connecting…</title></head>
+  <body>
+    Connecting…
+    <script>
+      (function () {
+        var params = new URLSearchParams(window.location.search)
+        var message = {
+          source: 'meta-oauth',
+          code: params.get('code'),
+          state: params.get('state'),
+          error: params.get('error_description') || params.get('error'),
+        }
+        if (window.opener) {
+          window.opener.postMessage(message, window.location.origin)
+        }
+        window.close()
+      })()
+    </script>
+  </body>
+</html>