refactor(confirm): use plain let for resolveRef instead of $state

resolveRef is never read reactively — it is only read synchronously
inside settle(). Using $state was misleading about the intent.

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

frontend/src/lib/services/confirm.svelte.ts

@@ -52,7 +52,7 @@ export interface ConfirmService {
 }
 
 export function createConfirmService(): ConfirmService {
-	let resolveRef: ((value: boolean) => void) | null = $state(null);
+	let resolveRef: ((value: boolean) => void) | null = null;
 	let options: ConfirmOptions | null = $state(null);
 
 	return {