feat: floating scroll nav buttons in scroll reader mode

Up/down chevron buttons fixed to the bottom-right of the viewport.
At the top of the chapter the up button becomes a Prev chapter link;
at the bottom the down button becomes an amber Next chapter link.
Hidden in focus mode (uses its own pill). Lifts above the audio
mini-player when it is active.

ui/src/routes/books/[slug]/chapters/[n]/+page.svelte

@@ -793,6 +793,67 @@
 	</div>
 {/if}
 
+<!-- ── Scroll mode floating nav buttons ──────────────────────────────────── -->
+{#if layout.readMode === 'scroll' && !layout.focusMode}
+{@const atTop = scrollProgress <= 0.01}
+{@const atBottom = scrollProgress >= 0.99}
+<div class="fixed right-4 {audioStore.active ? 'bottom-[5.5rem]' : 'bottom-8'} z-40 flex flex-col gap-2 transition-all">
+	<!-- Up button / Prev chapter -->
+	{#if atTop && data.prev}
+		<a
+			href="/books/{data.book.slug}/chapters/{data.prev}"
+			class="flex items-center justify-center w-10 h-10 rounded-full bg-(--color-surface-2)/90 backdrop-blur border border-(--color-border) shadow-lg text-(--color-brand) hover:bg-(--color-surface-3) transition-colors"
+			title="Previous chapter"
+			aria-label="Previous chapter"
+		>
+			<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>
+		</a>
+	{:else}
+		<button
+			type="button"
+			onclick={() => window.scrollBy({ top: -Math.round(window.innerHeight * 0.85), behavior: 'smooth' })}
+			disabled={atTop}
+			class="flex items-center justify-center w-10 h-10 rounded-full bg-(--color-surface-2)/90 backdrop-blur border border-(--color-border) shadow-lg text-(--color-muted) hover:text-(--color-text) hover:bg-(--color-surface-3) disabled:opacity-20 disabled:cursor-default transition-colors"
+			title="Scroll up"
+			aria-label="Scroll up"
+		>
+			<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}
+
+	<!-- Down button / Next chapter -->
+	{#if atBottom && data.next}
+		<a
+			href="/books/{data.book.slug}/chapters/{data.next}"
+			class="flex items-center justify-center w-10 h-10 rounded-full bg-(--color-brand) shadow-lg text-black hover:bg-(--color-brand-dim) transition-colors"
+			title="Next chapter"
+			aria-label="Next chapter"
+		>
+			<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="M19 9l-7 7-7-7"/>
+			</svg>
+		</a>
+	{:else}
+		<button
+			type="button"
+			onclick={() => window.scrollBy({ top: Math.round(window.innerHeight * 0.85), behavior: 'smooth' })}
+			disabled={atBottom && !data.next}
+			class="flex items-center justify-center w-10 h-10 rounded-full bg-(--color-surface-2)/90 backdrop-blur border border-(--color-border) shadow-lg text-(--color-muted) hover:text-(--color-text) hover:bg-(--color-surface-3) disabled:opacity-20 disabled:cursor-default transition-colors"
+			title="Scroll down"
+			aria-label="Scroll down"
+		>
+			<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="M19 9l-7 7-7-7"/>
+			</svg>
+		</button>
+	{/if}
+</div>
+{/if}
+
 <!-- ── Focus mode floating nav ───────────────────────────────────────────── -->
 {#if layout.focusMode}
 <div class="fixed {audioStore.active ? 'bottom-[4.5rem]' : 'bottom-6'} left-1/2 -translate-x-1/2 z-50 max-w-[calc(100vw-2rem)]">