fix: 修复 web-dist 分支 README 文件日期变量替换问题

- 移除 heredoc 的单引号，允许变量和命令替换
- 确保 Build Date 能正确显示实际构建时间
- 同时修复了 workflow 和测试脚本中的相同问题

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/test-web-dist.sh

@@ -85,7 +85,7 @@ test_build_and_push() {
     cp -r "$TEMP_DIR"/* .
     
     # 添加 README
-    cat > README.md << 'EOF'
+    cat > README.md << EOF
 # Claude Relay Service - Web Frontend Build
 
 This branch contains the pre-built frontend assets for Claude Relay Service.