fix(frontend): hide calendar popups on empty dates

Keep race popovers limited to dates with events so empty days no longer show misleading hover details.

Made-with: Cursor

frontend/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "calendar-run-frontend",
-  "version": "0.4.2",
+  "version": "0.4.3",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "calendar-run-frontend",
-      "version": "0.4.2",
+      "version": "0.4.3",
       "dependencies": {
         "react": "^18.3.1",
         "react-dom": "^18.3.1",