feat(preview): cross-browser scrollbar, keyboard nav, dev-mode client simulation, docs overhaul

preview/index.html:
- Fix scrollbar CSS: scoped webkit pseudo-elements + standards-track fallback, removed deprecated overflow:overlay
- Keyboard navigation: arrows cycle focus between Theme/Template/Client selects, up/down select within focused dropdown, d/m toggle viewport
- Dev/static mode detection: delayed static warning only when WebSocket absent, WebSocket errors only on disconnect (not on initial fail)
- iframe sandbox: removed static sandbox attr, applied dynamically only in dev mode (file: protocol rejects sandboxed srcdoc)
- Loading fix: removed double-toggle between init and render(); added 5s safety timeout
- Transform clean: returns HTML as-is; CSS stripping moved to server-side
- Dev disclaimer: blue info banner shown on WebSocket connect

tools/server/inliner.mjs:
- Added stripGmail() / stripOutlook() — server-side CSS property stripping for email client simulation

tools/server/server.mjs:
- Fixed WebSocket upgrade: use app.listen() instead of createServer(app).listen()
- Juice post-processing now generates three rendered.js variants (modern/gmail/outlook)
- Initial startup runs juice-only pass (avoids duplicate Go compilation)

docs/ (all 6 languages — en, zh-CN, zh-TW, ja, ko, ru):
- Image size limits: max 50KiB, recommended 10-20KiB
- Dev disclaimer: simulation cannot 100% reproduce every client
- Changed 'accurate' to 'relatively accurate' across all static-mode warnings
- Converted all plain blockquotes to GitHub admonitions ([!WARNING] / [!NOTE])

docs/CONTRIBUTING.zh-CN.md

@@ -42,9 +42,10 @@
 1. 先生成预览数据：`cd tools && go run . preview all`
 2. 在浏览器中打开 `preview/index.html` — 无需服务器
 
-> 静态 Gmail/Outlook 模拟仅供参考，使用 dev 模式可获得准确的 CSS 内联渲染。
+> [!WARNING]
+> 静态 Gmail/Outlook 模拟仅供参考，使用 dev 模式可获得相对准确的 CSS 内联渲染。
 
-### 开发服务器（实时重载 + CSS 内联）
+### 开发服务器（实时重载 + CSS 内联 + 客户端模拟）
 
 ```bash
 cd tools && go run . dev
@@ -53,6 +54,9 @@ cd tools && go run . dev
 
 修改 `.tmpl` 文件后自动重建并推送至浏览器。
 
+> [!NOTE]
+> Dev 模拟无法 100% 还原各个邮件客户端的渲染差异，仅供参考，请以实际效果为准。
+
 ### 集成测试
 
 将模板部署到 Gitea 实例，使用管理后台的测试邮件功能：