feat: add partial web platform support

[AmisKwok]

Summary / 摘要

Add partial Flutter Web support and ship it as 1.0.1, together with a CORS fix for the speed test, broader Windows VPN detection, and recovery of the app icons an earlier line-ending normalisation corrupted.
为插件新增 Flutter Web 部分支持并发布 1.0.1，同时修复 Web 端测速的 CORS 预检问题、扩展 Windows VPN 判定，并恢复此前换行归一化损坏的应用图标。

Changes / 变更

• Web platform (partial) — split the dart:io-dependent services into io/web variants behind conditional imports, add the web platform entry plus ZeroNetworkKitWeb, and add an example/web/ entry point / 将依赖 dart:io 的服务拆分为 io/web 两份实现并以条件导入隔离，新增 web 平台声明与 ZeroNetworkKitWeb，并补充 example/web/ 入口
• Connectivity uses connectivity_plus, ping degrades to an HTTPS round trip, DNS uses DNS-over-HTTPS; the speed test, quality score and benchmarks keep working unchanged / 连通性走 connectivity_plus，Ping 退化为 HTTPS 往返耗时，DNS 改用 DNS-over-HTTPS；测速、质量评分与基准测试行为不变
• Graceful degradation: native details report null, port checks return an "unavailable" result, and NetworkCapabilities.current() exposes the supported set / 优雅降级：原生详情返回 null，端口检测返回"不可用"结果，NetworkCapabilities.current() 暴露当前支持的能力集合
• Fix — drop the Cache-Control request header: it is not CORS-safelisted, so it forced an OPTIONS preflight that endpoints reject, surfacing as Failed to fetch on the web / 移除 Cache-Control 请求头：它不属于 CORS 安全头，会触发被端点拒绝的 OPTIONS 预检，在 Web 上表现为 Failed to fetch
• Fix — Windows now also recognises VPN adapters by driver name (TAP-Windows / Wintun / WireGuard / OpenVPN); IF_TYPE_PPP is deliberately excluded because PPPoE broadband shares that interface type / Windows 额外按驱动名识别 VPN 网卡（TAP-Windows / Wintun / WireGuard / OpenVPN）；有意排除 IF_TYPE_PPP，因为 PPPoE 宽带共用该接口类型
• Fix — restore the 31 app icons (Android / iOS / macOS / Windows) that an earlier normalisation rewrote as UTF-8, replacing non-ASCII bytes with U+FFFD / 恢复 31 个被此前归一化按 UTF-8 重写（非 ASCII 字节被替换为 U+FFFD）的应用图标
• Docs — document web support in both READMEs and on the docs site, and add the bilingual 1.0.1 changelog / 在两份 README 与文档站中说明 Web 支持，并补充双语的 1.0.1 变更日志

Context / 背景

The plugin previously could not compile for the web at all because its services imported dart:io, so every consumer targeting Flutter Web was blocked. Browsers also forbid raw TCP/UDP sockets and expose no VPN API, so web support is intentionally partial and documented as such instead of silently returning wrong values.
此前插件因服务层导入 dart:io 而完全无法编译到 Web，所有面向 Flutter Web 的使用方都被阻塞。浏览器同时禁止原生 TCP/UDP 套接字且不暴露 VPN 接口，因此 Web 支持有意设计为部分支持并明确文档化，而不是静默返回错误结果。

Checklist / 检查项

• Title follows Conventional Commits / 标题符合约定式提交
• CI checks pass after merge / 合入后 CI 通过

Test plan

• flutter analyze reports no issues and flutter test passes (49 tests) / flutter analyze 无问题，flutter test 通过（49 个用例）
• flutter build web succeeds in example/ / 在 example/ 下 flutter build web 构建成功
• Run the example on web: connectivity / DNS (DoH) / speed test work, port check reports "unavailable", isVpn stays false even with a VPN or proxy active / 在 Web 上运行 example：连通性、DNS（DoH）、测速可用，端口检测返回"不可用"，即使开启 VPN 或代理 isVpn 仍为 false
• Run the example on Windows/Android to confirm no regression in VPN detection and app icons / 在 Windows/Android 上运行 example，确认 VPN 判定与图标无回归

🤖 Generated with Zero Buddy