feat(confirm): add ConfirmService and ConfirmDialog with deferred-Promise pattern

- confirm.svelte.ts: context-based async service returning Promise<boolean>
- ConfirmDialog.svelte: native <dialog> element, reads service from context
- Concurrent calls return false immediately (guard at top of confirm())
- SSR-safe: confirm() returns Promise.resolve(false) on server
- getConfirmService() throws descriptive error outside provider tree
- 5 Vitest tests: confirm/cancel/Escape/concurrent/outside-provider all green

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

frontend/src/lib/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 />