feat: 添加账号支持选择学习平台

账号新增 platform 字段，登录/刷新课程/发现课程按平台分发；同时把 JS 脚本收进 script/（目录规范调整）。

script/desenc.js

@@ -0,0 +1,19 @@
+// Compute the site's strEnc(data, k1, k2, k3) using the original des.js.
+// usage: node script/desenc.js <data> [k1] [k2] [k3]
+const fs = require('fs');
+const path = require('path');
+const vm = require('vm');
+
+const desPath = path.join(__dirname, 'des.js');
+const code = fs.readFileSync(desPath, 'utf8');
+const ctx = { console };
+vm.createContext(ctx);
+vm.runInContext(code, ctx);
+
+const [data, k1 = '1', k2 = '2', k3 = '3'] = process.argv.slice(2);
+if (data === undefined) {
+  console.error('usage: node desenc.js <data> [k1] [k2] [k3]');
+  process.exit(2);
+}
+const out = ctx.strEnc(data, k1, k2, k3);
+process.stdout.write(String(out));