feat: 新增 web 端 — axum HTTP 服务 + Docker 化

新增 workspace crate :
- axum 0.8 + tokio 异步 HTTP 服务
- / → 嵌入式 HTML 页面（输入→实时预览→下载/复制）
- /api/qr?text=&level=M&margin=4&size=8 → PNG
- Dockerfile: rust-alpine 多阶段构建，镜像仅 ~12MB
- Cargo.toml: workspace 新增 web 成员

部署:
  docker build -t qrgen-web -f web/Dockerfile .
  docker run -p 3000:3000 qrgen-web

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

web/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "qrgen-web"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+authors.workspace = true
+
+[dependencies]
+qr-core = { path = "../core" }
+axum = "0.8"
+tokio = { version = "1", features = ["full"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tower-http = { version = "0.6", features = ["cors"] }