feat(api): add configurable landing page at /

Allow server operators to show new players how to install the game
client when they visit the server address in a browser. The page
content (title and HTML body) is fully configurable via config.json
and can be toggled on/off. Uses Go embed for a self-contained dark-
themed HTML template with zero new dependencies.

server/api/api_server.go

@@ -63,6 +63,7 @@ func (s *APIServer) Start() error {
 	r.HandleFunc("/character/export", s.ExportSave)
 	r.HandleFunc("/api/ss/bbs/upload.php", s.ScreenShot)
 	r.HandleFunc("/api/ss/bbs/{id}", s.ScreenShotGet)
+	r.HandleFunc("/", s.LandingPage)
 	r.HandleFunc("/health", s.Health)
 	r.HandleFunc("/version", s.Version)
 	handler := handlers.CORS(handlers.AllowedHeaders([]string{"Content-Type"}))(r)