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>

tools/go.mod

@@ -2,9 +2,10 @@ module gitea-mail-templates/tools
 
 go 1.21
 
+require github.com/urfave/cli/v2 v2.27.7
+
 require (
 	github.com/cpuguy83/go-md2man/v2 v2.0.7 // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
-	github.com/urfave/cli/v2 v2.27.7 // indirect
 	github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1 // indirect
 )