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])

README.md

@@ -89,7 +89,8 @@ cd tools && go run . preview all
 open preview/index.html        # no server needed
 ```
 
-> Gmail/Outlook simulation in static mode is approximate. Use dev mode for accurate CSS inlining.
+> [!WARNING]
+> Gmail/Outlook simulation in static mode is approximate. Use dev mode for relatively accurate CSS inlining.
 
 ### Dev Server (Live Reload + Juice CSS Inlining)
 
@@ -105,18 +106,21 @@ open http://localhost:3456
 | Go template rendering | ✅ | ✅ |
 | Theme/template switching | ✅ | ✅ |
 | Juice CSS inlining | — | ✅ |
-| Outlook `bgcolor` attrs | — | ✅ |
+| Gmail/Outlook CSS stripping | — | ✅ |
 | Live reload on save | — | ✅ |
 | Node.js required | — | ✅ |
 
+> [!NOTE]
+> Dev simulation cannot 100% reproduce every email client — for reference only; always verify against real clients.
+
 ### Features
 
 - Theme switcher — browse all 10 visual styles
 - Template switcher — all 11 email types
-- Client simulation — Modern, Gmail, Outlook, Raw Source
+- Client simulation — Modern, Gmail, Outlook, Raw Source (CSS stripping in dev mode)
 - Viewport toggle — Desktop 1386×780 / Mobile 390×780
 - Parameter panel — mock data per email type
-- Keyboard shortcuts — `←→` templates, `d`/`m` viewport
+- Keyboard shortcuts — `←→` tab between Theme/Template/Client, `↑↓` select within, `d`/`m` viewport
 
 ---