feat: 公开商品页重设计 + web 上传修复 + 二维码配置

- 公开商品页（/product/:uuid）全面重设计：全宽正方形图片轮播、
  左右滑动切图、点击放大全屏查看、商品参数含描述、页脚贴底
- 修复 Flutter web 文件上传无反应（path→bytes）
- 修复 web 路由空白页（usePathUrlStrategy + 单层 MaterialApp.router）
- 二维码 URL 改为从 STORAGE_PUBLIC_URL 环境变量读取
- 新增 PUBLIC_URL dart-define → AppConfig.publicBaseUrl
- 新增 CI/CD workflows + NAS runner compose 配置
- seed S001 补充商品描述字段

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches-ignore: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: backend/go.mod
+          cache-dependency-path: backend/go.sum
+
+      - name: Go tests
+        working-directory: backend
+        run: go test ./...
+
+      - uses: subosito/flutter-action@v2
+        with:
+          cache: true
+
+      - name: Flutter tests
+        working-directory: client
+        run: flutter test