feat: add xterm.js web terminal frontend

WebSocket-to-SSH proxy on :8080. Browser connects via xterm.js,
server bridges to localhost:2222 SSH. Single HTML file, CDN deps.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.go

@@ -7,6 +7,7 @@ import (
 	"github.com/tolelom/catacombs/game"
 	"github.com/tolelom/catacombs/server"
 	"github.com/tolelom/catacombs/store"
+	"github.com/tolelom/catacombs/web"
 )
 
 func main() {
@@ -20,7 +21,14 @@ func main() {
 
 	lobby := game.NewLobby()
 
-	log.Println("Catacombs server starting on :2222")
+	// Start web terminal server in background
+	go func() {
+		if err := web.Start(":8080", 2222); err != nil {
+			log.Printf("Web server error: %v", err)
+		}
+	}()
+
+	log.Println("Catacombs server starting — SSH :2222, Web :8080")
 	if err := server.Start("0.0.0.0", 2222, lobby, db); err != nil {
 		log.Fatal(err)
 	}