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.

web/static/input.css

@@ -1029,6 +1029,51 @@
     background-color: color-mix(in srgb, currentColor 6%, transparent);
   }
 
+  /* Page thumbnails grid (contents drawer, fixed-layout) */
+  .reader-thumb-grid {
+    display: grid;
+    grid-template-columns: repeat(3, 1fr);
+    gap: 0.5rem;
+  }
+  .reader-thumb {
+    position: relative;
+    border-radius: 0.375rem;
+    overflow: hidden;
+    border: 2px solid transparent;
+    padding: 0;
+    background-color: color-mix(in srgb, currentColor 6%, transparent);
+  }
+  .reader-thumb:hover {
+    border-color: color-mix(in srgb, currentColor 30%, transparent);
+  }
+  .reader-thumb.active {
+    border-color: #3b82f6;
+  }
+  .reader-thumb-img {
+    aspect-ratio: 3 / 4;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    overflow: hidden;
+  }
+  .reader-thumb-img canvas,
+  .reader-thumb-img img {
+    max-width: 100%;
+    max-height: 100%;
+    object-fit: contain;
+  }
+  .reader-thumb-num {
+    position: absolute;
+    bottom: 0.2rem;
+    right: 0.35rem;
+    font-size: 0.65rem;
+    line-height: 1;
+    padding: 0.15rem 0.3rem;
+    border-radius: 0.25rem;
+    background-color: rgba(0, 0, 0, 0.55);
+    color: #ffffff;
+  }
+
   /* In-book search */
   .reader-search-bar {
     display: flex;