fix: stringify config values before PATCH to avoid 422 (number vs string)

frontend/src/routes/admin/+page.svelte

@@ -82,7 +82,7 @@
 			const changes: Record<string, string> = {};
 			for (const key of Object.keys(configDraft)) {
 				if (configDraft[key] !== config[key]) {
-					changes[key] = configDraft[key];
+					changes[key] = String(configDraft[key]);
 				}
 			}
 			if (Object.keys(changes).length === 0) {