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.svelte.ts

@@ -0,0 +1,100 @@
+/**
+ * Context-based confirmation service. Provides an async `confirm()` function
+ * that any component can call without managing its own modal state.
+ *
+ * ## Setup
+ * Mount `<ConfirmDialog>` once in the root `+layout.svelte` — it sets up the context
+ * automatically. Then call `getConfirmService()` from any descendant component.
+ *
+ * ## Usage in event handlers
+ * ```typescript
+ * import { getConfirmService } from '$lib/services/confirm.svelte.js';
+ * const { confirm } = getConfirmService();
+ *
+ * async function handleDelete() {
+ *   const ok = await confirm({ title: m.confirm_delete_title(), destructive: true });
+ *   if (ok) doDelete();
+ * }
+ * ```
+ *
+ * ## Usage with use:enhance
+ * ```svelte
+ * <form use:enhance={async ({ cancel }) => {
+ *   const ok = await confirm({ title: m.confirm_delete_title(), destructive: true });
+ *   if (!ok) cancel();
+ * }}>
+ * ```
+ */
+import { getContext, setContext } from 'svelte';
+import { browser } from '$app/environment';
+
+export const CONFIRM_KEY = Symbol('confirm');
+
+export interface ConfirmOptions {
+	title: string;
+	body?: string;
+	/** Defaults to m.btn_confirm() ("Bestätigen") */
+	confirmLabel?: string;
+	/** Defaults to m.btn_cancel() ("Abbrechen") */
+	cancelLabel?: string;
+	/** Uses danger color for confirm button. Defaults to false. */
+	destructive?: boolean;
+	/** Close when clicking outside the dialog. Defaults to !destructive. */
+	closeOnBackdrop?: boolean;
+}
+
+export interface ConfirmService {
+	confirm(opts: ConfirmOptions): Promise<boolean>;
+	/** Read by ConfirmDialog to render the current dialog. Internal use only. */
+	readonly options: ConfirmOptions | null;
+	/** Called by ConfirmDialog when the user makes a choice. Internal use only. */
+	settle(value: boolean): void;
+}
+
+export function createConfirmService(): ConfirmService {
+	let resolveRef: ((value: boolean) => void) | null = $state(null);
+	let options: ConfirmOptions | null = $state(null);
+
+	return {
+		confirm(opts: ConfirmOptions): Promise<boolean> {
+			if (!browser) return Promise.resolve(false);
+			// Concurrent call while dialog is already open — reject immediately.
+			if (resolveRef !== null) return Promise.resolve(false);
+			options = opts;
+			return new Promise((r) => {
+				resolveRef = r;
+			});
+		},
+
+		get options() {
+			return options;
+		},
+
+		settle(value: boolean): void {
+			options = null;
+			const r = resolveRef;
+			resolveRef = null;
+			r?.(value);
+		}
+	};
+}
+
+export function provideConfirmService(): ConfirmService {
+	const service = createConfirmService();
+	setContext(CONFIRM_KEY, service);
+	return service;
+}
+
+export function getConfirmService(): ConfirmService {
+	// Outside component init, getContext either returns undefined or throws a Svelte error.
+	// Either way, map it to our descriptive developer error.
+	let service: ConfirmService | undefined;
+	try {
+		service = getContext<ConfirmService>(CONFIRM_KEY);
+	} catch {
+		throw new Error('ConfirmService not found — mount <ConfirmDialog> in +layout.svelte');
+	}
+	if (!service)
+		throw new Error('ConfirmService not found — mount <ConfirmDialog> in +layout.svelte');
+	return service;
+}