feat(reader): page thumbnails tab + reliable PDF contents

Investigation: the contents drawer read book.toc, which makePDF
builds from pdf.getOutline() — verified against the real library PDF
(Head First SQL) through the exact vendored pdf.js build AND the exact
range transport the browser uses: 18 chapter entries come back. So
the source is right; manga-scan PDFs and CBZs simply have no embedded
outline, which made Contents look broken exactly where users expect
page-based navigation.

- TOC now populates eagerly right after the book opens (toggle-time
  lazy population removed), so an existing outline can never silently
  miss due to timing; the drawer keeps the honest empty-state text
  for books without outlines.
- New 'Pages' tab in the contents drawer for fixed-layout books:
  a Kavita-style thumbnail grid (3-up, current page highlighted and
  scrolled into view, click to jump — recorded on the back-to-
  location stack). Thumbnails render client-side: PDFs via the
  in-memory pdf.js document (small viewport render, Alpine.raw
  unwrap); comics via the page's image blob drawn down to a 110px
  canvas, then unloading the full-size blob so thumbnailling doesn't
  hoard page images. Lazy via IntersectionObserver scoped to the
  drawer's scroll container (200px margin), canvases cached at module
  level so revisits are instant; failures warn in console and allow
  retry. The backend /readers/thumbnails endpoint turned out to be an
  empty stub, so nothing server-side was worth wiring.

templates/reader.templ

@@ -521,8 +521,18 @@ templ drawerHeader(title string) {
 
 templ ReaderTOCDrawer() {
 	@drawerHeader("Contents")
+	<!-- Fixed-layout books get a Pages (thumbnails) tab; outline TOC may be
+	     absent in manga-scan PDFs and CBZs. -->
+	<div class="reader-tabs" x-show="isFixedLayout">
+		<button :class="tocTab === 'contents' ? 'active' : ''" @click="tocTab = 'contents'">
+			Contents <span class="reader-tab-count" x-text="tocItems.length">0</span>
+		</button>
+		<button :class="tocTab === 'pages' ? 'active' : ''" @click="tocTab = 'pages'; initPageThumbs()">
+			Pages <span class="reader-tab-count" x-text="pageThumbList.length || ''">0</span>
+		</button>
+	</div>
 	<div class="reader-drawer-body">
-		<nav id="toc-list" class="space-y-1">
+		<nav id="toc-list" class="space-y-1" x-show="tocTab === 'contents' || !isFixedLayout">
 			<template x-for="item in tocItems" :key="item.href">
 				<a
 					href="#"
@@ -536,6 +546,22 @@ templ ReaderTOCDrawer() {
 				<p class="text-sm py-2" style="color: var(--text-secondary)">No table of contents</p>
 			</template>
 		</nav>
+		<div id="page-thumb-grid" class="reader-thumb-grid" x-show="isFixedLayout && tocTab === 'pages'">
+			<template x-for="p in pageThumbList" :key="p.index">
+				<button
+					class="reader-thumb"
+					:class="p.index === currentPageIndex ? 'active' : ''"
+					@click="goToPage(p.index)"
+					:title="'Page ' + (p.index + 1)"
+				>
+					<div class="reader-thumb-img" :data-page="p.index"></div>
+					<span class="reader-thumb-num" x-text="p.index + 1">1</span>
+				</button>
+			</template>
+			<template x-if="pageThumbList.length === 0">
+				<p class="text-sm py-2" style="color: var(--text-secondary)">No pages</p>
+			</template>
+		</div>
 	</div>
 }