refactor: move shared utilities to lib/shared/ sub-packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/services/confirm.test-host.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+import { provideConfirmService, type ConfirmService } from './confirm.svelte.js';
+import ConfirmDialog from '$lib/components/ConfirmDialog.svelte';
+
+let { onReady }: { onReady: (service: ConfirmService) => void } = $props();
+
+const service = provideConfirmService();
+onReady(service);
+</script>
+
+<ConfirmDialog />