fix: replace $effect with onMount for initial data load

$effect re-runs on any reactive state change — loadData/loadConfig both
write state, creating an infinite loop that hung the page under load.

data/units.json

@@ -117,7 +117,9 @@
       "id": "another-group",
       "label": "Another Group",
       "baseUnitId": "",
-      "toUniversal": 1
+      "toUniversal": 1,
+      "alwaysShowLabel": false,
+      "hidden": true
     }
   ]
 }