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])

AGENTS.md

@@ -49,9 +49,11 @@ docs/               # Multi-language documentation
 ### Preview System
 - `preview/index.html` loads `preview/rendered.js` (pre-rendered by Go) and displays in iframes
 - Supports theme/template switching, client simulation (Modern/Gmail/Outlook/Raw), and viewport toggle (Desktop 1386x780 / Mobile 390x780)
+- Keyboard navigation: `←→` cycles focus between Theme/Template/Client selects, `↑↓` selects within the focused dropdown, `d`/`m` toggles viewport
 - `REGISTRY` and `PARAMS` are auto-generated from `templates_config.json` — no manual syncing needed
-- Static preview (open `index.html` directly) provides approximate client simulation
-- Dev server (`go run . dev`) provides accurate rendering via Juice CSS inlining + live reload
+- Static preview (open `index.html` directly) — all client modes show the same HTML; simulation warning is displayed
+- Dev server (`go run . dev`) — applies Juice CSS inlining server-side, generates three `rendered.js` variants (modern/gmail/outlook) with client-specific CSS stripping; includes live reload via WebSocket
+- Dev mode shows an info notice: simulation cannot 100% reproduce every email client — always verify against real clients
 
 ### Build Tool
 - `tools/tools.go` is the main entry point for the modular CLI