feat: merge page nav into focus mode floating pill

In paginated + focus mode there were two separate UI elements: an inline
Prev/counter/Next bar and a floating chapter-nav pill. Merged into one:

- ‹ Ch.N | ‹ (page) N/M (page) › | × Exit focus | Ch.N ›
- Inline page bar + hint text are now hidden when focusMode is active
- Floating pill grows to include page controls only in paginated mode;
  scroll mode pill is unchanged (just chapter nav + exit)
- Added max-w-[calc(100vw-2rem)] so pill never overflows on small screens

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

@@ -567,7 +567,8 @@
 		</div>
 	</div>
 
-	<!-- Page indicator + nav -->
+	<!-- Page indicator + nav (hidden in focus mode — shown in floating pill instead) -->
+	{#if !layout.focusMode}
 	<div class="flex items-center justify-between mt-4 select-none">
 		<button
 			type="button"
@@ -596,6 +597,7 @@
 		</button>
 	</div>
 	<p class="text-center text-xs text-(--color-muted)/40 mt-2">Tap left/right · Arrow keys · Space</p>
+	{/if}
 {:else}
 	<!-- ── Scroll reader ──────────────────────────────────────────────── -->
 	<div class="prose-chapter mt-8 {layout.paraStyle === 'indented' ? 'para-indented' : ''}">
@@ -649,12 +651,13 @@
 
 <!-- ── 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">
+<div class="fixed {audioStore.active ? 'bottom-[4.5rem]' : 'bottom-6'} left-1/2 -translate-x-1/2 z-50 max-w-[calc(100vw-2rem)]">
 	<div class="flex items-center divide-x divide-(--color-border) rounded-full bg-(--color-surface-2)/95 backdrop-blur border border-(--color-border) shadow-lg text-xs text-(--color-muted) overflow-hidden">
+		<!-- Prev chapter -->
 		{#if data.prev}
 			<a
 				href="/books/{data.book.slug}/chapters/{data.prev}"
-				class="flex items-center gap-1 px-3 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) transition-colors"
+				class="flex items-center gap-1 px-3 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) transition-colors shrink-0"
 				aria-label="Previous chapter"
 			>
 				<svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
@@ -663,10 +666,41 @@
 				{m.reader_chapter_n({ n: String(data.prev) })}
 			</a>
 		{/if}
+
+		<!-- Page prev / counter / next (paginated mode only) -->
+		{#if layout.readMode === 'paginated'}
+			<button
+				type="button"
+				onclick={() => { if (pageIndex > 0) pageIndex--; }}
+				disabled={pageIndex === 0}
+				class="flex items-center justify-center px-2.5 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) disabled:opacity-30 transition-colors shrink-0"
+				aria-label="Previous page"
+			>
+				<svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+					<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 19l-7-7 7-7"/>
+				</svg>
+			</button>
+			<span class="px-2.5 py-2 tabular-nums text-(--color-muted) shrink-0 select-none">
+				{pageIndex + 1}<span class="opacity-40">/</span>{totalPages}
+			</span>
+			<button
+				type="button"
+				onclick={() => { if (pageIndex < totalPages - 1) pageIndex++; }}
+				disabled={pageIndex === totalPages - 1}
+				class="flex items-center justify-center px-2.5 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) disabled:opacity-30 transition-colors shrink-0"
+				aria-label="Next page"
+			>
+				<svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+					<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 5l7 7-7 7"/>
+				</svg>
+			</button>
+		{/if}
+
+		<!-- Exit focus -->
 		<button
 			type="button"
 			onclick={() => setLayout('focusMode', false)}
-			class="flex items-center gap-1 px-3 py-2 hover:text-(--color-brand) hover:bg-(--color-surface-3) transition-colors"
+			class="flex items-center gap-1 px-3 py-2 hover:text-(--color-brand) hover:bg-(--color-surface-3) transition-colors shrink-0"
 			aria-label="Exit focus mode"
 		>
 			<svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
@@ -674,10 +708,12 @@
 			</svg>
 			Exit focus
 		</button>
+
+		<!-- Next chapter -->
 		{#if data.next}
 			<a
 				href="/books/{data.book.slug}/chapters/{data.next}"
-				class="flex items-center gap-1 px-3 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) transition-colors"
+				class="flex items-center gap-1 px-3 py-2 hover:text-(--color-text) hover:bg-(--color-surface-3) transition-colors shrink-0"
 				aria-label="Next chapter"
 			>
 				{m.reader_chapter_n({ n: String(data.next) })}