Add type:module to root and shared package.json

package.json

@@ -1,6 +1,7 @@
 {
   "name": "mummy-card-game",
   "version": "1.0.0",
+  "type": "module",
   "description": "Multiplayer Egyptian-themed card game with real-time gameplay",
   "scripts": {
     "install:all": "cd client && npm install && cd ../server && npm install",

shared/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}