fix(discover): guard active card with {#if currentBook} to fix TS errors; use $state for cardEl bind

ui/src/routes/discover/+page.svelte

@@ -114,7 +114,7 @@
 	);
 	const rotation = $derived(isDragging ? Math.max(-18, Math.min(18, offsetX / 12)) : 0);
 
-	let cardEl: HTMLDivElement | null = null;
+	let cardEl = $state<HTMLDivElement | null>(null);
 
 	function onPointerDown(e: PointerEvent) {
 		if (animating || !currentBook) return;
@@ -446,6 +446,7 @@
 			{/if}
 
 			<!-- Active card -->
+			{#if currentBook}
 			<div
 				bind:this={cardEl}
 				class="absolute inset-0 rounded-2xl overflow-hidden shadow-2xl cursor-grab active:cursor-grabbing z-10"
@@ -522,6 +523,7 @@
 					<span class="text-(--color-muted) font-black text-lg tracking-widest">NOPE</span>
 				</div>
 			</div>
+			{/if}
 		</div>
 
 		<!-- Action buttons -->