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/README.zh-CN.md

@@ -56,12 +56,27 @@ cd tools && go run . preview all
 ```
 然后打开 `preview/index.html`。
 
-**开发服务器（实时重载 + Juice CSS 内联）：**
+> [!WARNING]
+> 静态模式下 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 + Juice CSS 内联 + Gmail/Outlook 模拟）：**
 ```bash
 cd tools && go run . dev     # 需要 Node.js
 # → http://localhost:3456
 ```
 
+> [!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。
+
+| 功能 | 静态 | Dev |
+|-----------|--------|-----|
+| Go 模板渲染 | ✅ | ✅ |
+| 主题/模板切换 | ✅ | ✅ |
+| Juice CSS 内联 | — | ✅ |
+| Gmail/Outlook CSS 剥离 | — | ✅ |
+| 实时重载 | — | ✅ |
+| 需要 Node.js | — | ✅ |
+
 ---
 
 ## 兼容性