feat: add server=side cfg and default theme

engine/handlers/server_cfg.go

@@ -0,0 +1,18 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/gabehf/koito/internal/cfg"
+	"github.com/gabehf/koito/internal/utils"
+)
+
+type ServerConfig struct {
+	DefaultTheme string `json:"default_theme"`
+}
+
+func GetCfgHandler() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		utils.WriteJSON(w, http.StatusOK, ServerConfig{DefaultTheme: cfg.DefaultTheme()})
+	}
+}