fix: gate platform implementations on dart.library.io for the Web build

[AmisKwok]

Summary / 摘要

Gate the platform-specific implementations on dart.library.io so the Web build no longer resolves dart:io. / 将各平台实现改为按 dart.library.io 条件引入，使 Web 构建不再解析到 dart:io。

Changes / 变更

• Flip the conditional exports in ping_service.dart, connectivity_service.dart, dns_service.dart, port_service.dart and network_capabilities.dart: the browser-based variants are now the default, and the dart:io variants are gated behind dart.library.io. / 反转 5 处条件导出：浏览器版本成为默认，dart:io 版本改为按 dart.library.io 命中时启用。
• Bump the version to 1.0.4 across pubspec.yaml, both podspecs, README.md, README_zh.md and CHANGELOG.md. / 版本升级至 1.0.4（pubspec、iOS/macOS podspec、两份 README、CHANGELOG）。

Context / 背景

The conditional exports previously defaulted to the dart:io variants and only switched to the browser-based variants when dart.library.html was defined. Any environment that does not define dart.library.html therefore still resolved the dart:io variants and pulled dart:io into the Web build. Making the browser-based variants the default and gating the dart:io variants behind dart.library.io leaves the fallback dart:io-free, while runtime behaviour on every supported platform stays exactly the same. / 此前条件导出默认落到 dart:io 版本，仅在定义了 dart.library.html 时切到浏览器版本；未定义该库的环境仍会解析到 dart:io 版本，从而把 dart:io 带入 Web 构建。改为浏览器版本兜底、原生版本按 dart.library.io 引入后，兜底路径不含 dart:io，且各平台运行时行为完全不变。

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

• dart format --set-exit-if-changed ., flutter analyze and flutter test all pass locally (49 tests). / 本地格式化校验、静态分析与测试均通过（49 个用例）。
• After publishing 1.0.4, confirm the Web platform is reported as compatible with the WASM runtime. / 发布 1.0.4 后确认 Web 平台被判定为兼容 WASM 运行时。

🤖 Generated with Zero Buddy