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
     }
   ]
 }

src/routes/admin/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { getContext } from 'svelte';
+  import { getContext, onMount } from 'svelte';
   import type { Unit, Group, UnitsData } from '$lib/types';
 
   const adminPath = getContext<string>('adminPath') ?? 'admin';
@@ -239,7 +239,7 @@
     expandedGroups = new Set([...expandedGroups, ...groups.map((g) => g.id)]);
   }
 
-  $effect(() => { loadData(); loadConfig(); });
+  onMount(() => { loadData(); loadConfig(); });
 
   async function extractError(res: Response, fallback: string): Promise<string> {
     const body = await res.json().catch(() => ({}));