fix: make ffi-napi optional to prevent CI build failures

- Move ffi-napi, ref-napi, ref-struct-napi to optionalDependencies
- Prevents compilation errors in CI with Node.js 20.x
- Embedded mode still works when dependencies are installed
- Server mode (primary use case) works without FFI dependencies

Author: sushanthpy

package.json

@@ -79,14 +79,14 @@
     "typescript": "^5.3.3"
   },
   "dependencies": {
-    "@grpc/grpc-js": "^1.14.3",
-    "@grpc/proto-loader": "^0.8.0",
-    "uuid": "^9.0.0",
+    "@grpc/grpc-js": "^1.12.4",
+    "@grpc/proto-loader": "^0.7.15",
+    "uuid": "^9.0.0"
+  },
+  "optionalDependencies": {
+    "posthog-node": "^4.18.0",
     "ffi-napi": "^4.0.3",
     "ref-napi": "^3.0.3",
     "ref-struct-napi": "^1.1.1"
-  },
-  "optionalDependencies": {
-    "posthog-node": "^4.18.0"
   }
-}
+}