📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/core-js/modules/es.uint8-array.from-base64.js

@@ -0,0 +1,29 @@
+'use strict';
+var $ = require('../internals/export');
+var globalThis = require('../internals/global-this');
+var arrayFromConstructorAndList = require('../internals/array-from-constructor-and-list');
+var $fromBase64 = require('../internals/uint8-from-base64');
+
+var Uint8Array = globalThis.Uint8Array;
+
+var INCORRECT_BEHAVIOR_OR_DOESNT_EXISTS = !Uint8Array || !Uint8Array.fromBase64 || !function () {
+  // Webkit not throw an error on odd length string
+  try {
+    Uint8Array.fromBase64('a');
+    return;
+  } catch (error) { /* empty */ }
+  try {
+    Uint8Array.fromBase64('', null);
+  } catch (error) {
+    return true;
+  }
+}();
+
+// `Uint8Array.fromBase64` method
+// https://tc39.es/ecma262/#sec-uint8array.frombase64
+if (Uint8Array) $({ target: 'Uint8Array', stat: true, forced: INCORRECT_BEHAVIOR_OR_DOESNT_EXISTS }, {
+  fromBase64: function fromBase64(string /* , options */) {
+    var result = $fromBase64(string, arguments.length > 1 ? arguments[1] : undefined, null, 0x1FFFFFFFFFFFFF);
+    return arrayFromConstructorAndList(Uint8Array, result.bytes);
+  }
+});