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>

AGENTS.md

@@ -48,12 +48,11 @@ 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
+- Supports theme/template switching, view mode (Modern/Source), and viewport toggle (Desktop 1386x780 / Mobile 390x780)
+- Keyboard navigation: `←→` cycles focus between Theme/Template/View 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) — 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
+- Static preview (open `index.html` directly) — works via `file://` protocol with Modern and Source views
+- Dev server (`go run . dev`) — pure Go HTTP server with SSE live reload; watches `themes/` for `.tmpl` changes, re-renders in-process, and pushes reload events to the browser
 
 ### Build Tool
 - `tools/tools.go` is the main entry point for the modular CLI
@@ -62,7 +61,7 @@ docs/               # Multi-language documentation
 - `tools/config/` handles config loading and data flattening
 - `tools/preview/` implements the rendering engine (template funcs, locale, engine, markSafeHTML)
 - `tools/cli/` implements CLI subcommands using `github.com/urfave/cli/v2`
-- `tools/server/` Node.js dev server with Juice CSS inlining and live reload (Express + WebSocket + fs.watch)
+- `tools/preview/server.go` pure Go dev server with SSE live reload and in-process template re-rendering
 - Uses Go's native `html/template` package for template rendering
 
 ## Commit Conventions