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/components/ConfirmDialog.svelte

@@ -0,0 +1,61 @@
+<script lang="ts">
+import { getConfirmService } from '$lib/services/confirm.svelte.js';
+import * as m from '$lib/paraglide/messages.js';
+
+// Context must already be set by the parent layout via provideConfirmService().
+const service = getConfirmService();
+
+let dialogEl: HTMLDialogElement;
+
+$effect(() => {
+	if (service.options) {
+		dialogEl.showModal();
+	} else {
+		dialogEl.close();
+	}
+});
+</script>
+
+<dialog
+	bind:this={dialogEl}
+	class="max-w-sm rounded-sm border border-line bg-surface p-6 shadow-lg"
+	aria-labelledby="confirm-title"
+	oncancel={(e) => {
+		e.preventDefault();
+		service.settle(false);
+	}}
+	onclick={(e) => {
+		const opts = service.options;
+		if (!opts) return;
+		const closeOnBackdrop = opts.closeOnBackdrop ?? !opts.destructive;
+		if (closeOnBackdrop && e.target === dialogEl) {
+			service.settle(false);
+		}
+	}}
+>
+	{#if service.options}
+		{@const opts = service.options}
+		<h2 id="confirm-title" class="mb-2 font-serif text-lg text-ink">{opts.title}</h2>
+		{#if opts.body !== undefined}
+			<p class="mb-6 text-sm text-ink-2">{opts.body}</p>
+		{/if}
+		<div class="flex items-center justify-end gap-3">
+			<button
+				type="button"
+				class="min-h-[44px] rounded-sm border border-line px-4 py-2 text-sm font-medium text-ink-2 transition-colors hover:bg-muted"
+				onclick={() => service.settle(false)}
+			>
+				{opts.cancelLabel ?? m.btn_cancel()}
+			</button>
+			<button
+				type="button"
+				class="min-h-[44px] rounded-sm px-4 py-2 text-sm font-medium transition-colors {opts.destructive
+					? 'bg-danger text-danger-fg'
+					: 'bg-primary text-primary-fg'}"
+				onclick={() => service.settle(true)}
+			>
+				{opts.confirmLabel ?? m.btn_confirm()}
+			</button>
+		</div>
+	{/if}
+</dialog>