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.ko.md

@@ -42,12 +42,15 @@ cd tools && go run . preview all
 ```
 그런 다음 `preview/index.html` 열기.
 
-**개발 서버 (실시간 리로드 + Juice CSS 인라인):**
+**개발 서버 (실시간 리로드 + Juice CSS 인라인 + 클라이언트 시뮬레이션):**
 ```bash
 cd tools && go run . dev     # Node.js 필요
 # → http://localhost:3456
 ```
 
+> [!NOTE]
+> Dev 시뮬레이션은 각 이메일 클라이언트의 렌더링을 완전히 재현할 수 없습니다 — 참고용입니다. 실제 클라이언트에서 반드시 확인하세요.
+
 ## 호환성
 
 - **Gitea 1.21+**, 100% 호환, 내장 함수만 사용