build(admin): track the SPA build output so git deploys carry it

assets/admin/build was gitignored, so servers deployed by git pull kept
running a stale admin bundle — the icon picker integration in the tabbar
and floating-button settings pages never reached them. Track the build
directory (rebuild with: cd assets/admin && npm run build) and commit the
current bundle containing the universal icon picker.

assets/admin/build/index.html

@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html lang="zh-CN">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+  <title>AKSRT 主题设置</title>
+  <script type="module" crossorigin src="./index.js"></script>
+  <link rel="modulepreload" crossorigin href="./vendor-vue.BxZIRU0_.js">
+  <link rel="modulepreload" crossorigin href="./vendor-misc.bELTaXdf.js">
+  <link rel="modulepreload" crossorigin href="./vendor-naive.CKYCZmXD.js">
+  <link rel="stylesheet" crossorigin href="./index.css">
+</head>
+<body>
+  <div id="aksrt-admin-app"></div>␍
+</body>
+</html>