refactor(preview): remove juice/Node.js, simplify to pure Go dev server with SSE

- Delete tools/server/ (Node.js, juice, inliner.mjs, server.mjs, node_modules)
- Create tools/preview/server.go — pure Go dev server with SSE + file polling
- Rewrite tools/cli/dev.go — start Go server in-process, no Node.js required
- Simplify preview/index.html — View dropdown (Modern/Source), remove Client
  Simulation panel, remove Gmail/Outlook logic, WebSocket → SSE
- Update all 14 documentation files (README, CONTRIBUTING, AGENTS + translations)
  to reflect pure Go dev server and simplified preview

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

docs/README.zh-TW.md

@@ -62,14 +62,12 @@ cd tools && go run . preview all
 ```
 然後開啟 `preview/index.html`。
 
-**開發伺服器（即時重載 + Juice CSS 內聯 + 客戶端模擬）：**
+**開發伺服器（即時重載）：**
 ```bash
-cd tools && go run . dev     # 需要 Node.js
+cd tools && go run . dev 
 # → http://localhost:3456
 ```
 
-> [!NOTE]
-> Dev 模擬無法 100% 還原各個郵件用戶端的渲染差異，僅供參考，請以實際效果為準。
 
 ---