Add scroll-to-top button on browse page and mobile nav drawer

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

@@ -152,6 +152,16 @@
 
 	// Admin: refresh catalogue
 	let refreshing = $state(false);
+
+	// ── Scroll-to-top button ─────────────────────────────────────────────────
+	let showScrollTop = $state(false);
+	$effect(() => {
+		function onScroll() {
+			showScrollTop = window.scrollY > 400;
+		}
+		window.addEventListener('scroll', onScroll, { passive: true });
+		return () => window.removeEventListener('scroll', onScroll);
+	});
 </script>
 
 <svelte:head>
@@ -559,3 +569,17 @@
 		<p class="text-center text-zinc-600 text-xs mt-8 pb-4">All novels loaded</p>
 	{/if}
 {/if}
+
+<!-- Scroll-to-top button -->
+{#if showScrollTop}
+	<button
+		onclick={() => window.scrollTo({ top: 0, behavior: 'smooth' })}
+		class="fixed bottom-6 right-6 z-50 p-3 rounded-full bg-zinc-800 border border-zinc-600 text-zinc-300 shadow-lg hover:bg-zinc-700 hover:text-zinc-100 transition-colors"
+		title="Back to top"
+		aria-label="Scroll to top"
+	>
+		<svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+			<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M5 15l7-7 7 7" />
+		</svg>
+	</button>
+{/if}