feat: implement email validation, CI/CD pipeline, migration history, and web templates

Email validation:
- Replace @/. check with net/mail.ParseAddress on register
- Add size limit check (max 254 chars, RFC 5321)

CI/CD Pipeline:
- Add .gitea/workflows/ci.yml (lint → test → build → docker push)
- Registry: gitea.mrixs.me/Mrixs/MrixsCraft-server
- Push only on main branch

Database:
- Add migrations/002_migration_history.sql (tracking applied migrations)
- Add migrations/README.md (manual apply instructions)

Web Templates:
- Add base.html with Minecraft-themed layout (dark + green accent)
- Add index.html, login.html, register.html with POST forms
- Rewrite templates.go for data-driven rendering with pageData struct
- Fallback placeholder preserved when templates dir missing

cmd/server/main.go

@@ -81,7 +81,7 @@ func main() {
 		Handler: handler,
 	}
 
-	// Graceful shutdown.
+	// Graceful shutdown on SIGINT/SIGTERM.
 	done := make(chan os.Signal, 1)
 	signal.Notify(done, syscall.SIGINT, syscall.SIGTERM)