📦 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/internals/math-float-round.js

@@ -0,0 +1,19 @@
+'use strict';
+var sign = require('../internals/math-sign');
+var roundTiesToEven = require('../internals/math-round-ties-to-even');
+
+var abs = Math.abs;
+
+var EPSILON = 2.220446049250313e-16; // Number.EPSILON
+
+module.exports = function (x, FLOAT_EPSILON, FLOAT_MAX_VALUE, FLOAT_MIN_VALUE) {
+  var n = +x;
+  var absolute = abs(n);
+  var s = sign(n);
+  if (absolute < FLOAT_MIN_VALUE) return s * roundTiesToEven(absolute / FLOAT_MIN_VALUE / FLOAT_EPSILON) * FLOAT_MIN_VALUE * FLOAT_EPSILON;
+  var a = (1 + FLOAT_EPSILON / EPSILON) * absolute;
+  var result = a - (a - absolute);
+  // eslint-disable-next-line no-self-compare -- NaN check
+  if (result > FLOAT_MAX_VALUE || result !== result) return s * Infinity;
+  return s * result;
+};