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])

CONTRIBUTING.md

@@ -49,9 +49,10 @@ Documentation updates, preview screenshots, installation guides, and translation
 2. Open `preview/index.html` directly in a browser — no server needed
 3. Use the theme switcher, template selector, and client mode toggles
 
-> Static Gmail/Outlook simulation is approximate. Use dev mode for accurate rendering.
+> [!WARNING]
+> Static Gmail/Outlook simulation is approximate. Use dev mode for relatively accurate rendering.
 
-### Dev Server (Live Reload + CSS Inlining)
+### Dev Server (Live Reload + CSS Inlining + Client Simulation)
 
 ```bash
 cd tools && go run . dev
@@ -60,10 +61,13 @@ cd tools && go run . dev
 
 - Watches `themes/**/*.tmpl` — auto-rebuilds on save
 - Runs [Juice](https://github.com/Automattic/juice) to inline `<style>` into `style=""` attributes
-- Adds Outlook-compatible `bgcolor`/`width` HTML attributes
+- Generates three `rendered.js` variants: modern (Juice only), Gmail (Juice + CSS strip), Outlook (Juice + aggressive CSS strip)
 - Pushes live reload to browser via WebSocket
 - Terminal output: `themes/aurora/mail/repo/release.tmpl edited` → `[rebuild] done in 480ms`
 
+> [!NOTE]
+> Dev simulation cannot 100% reproduce every email client — for reference only; always verify against real clients.
+
 ### Integration Testing
 
 Deploy the templates to a Gitea instance and use the admin test email feature: