Reader: contain img inside aspect-ratio placeholder

Prior rendering let <img> flow naturally (h-auto), so subpixel aspect
mismatches between DB dims and natural file dims caused img to over-
flow the placeholder div — manifesting as gaps or content overlap
between consecutive pages. Switching to absolute inset-0 +
object-contain pins the img to div dimensions regardless.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

components/PageReader.tsx

@@ -659,7 +659,7 @@ export function PageReader({
                         src={url}
                         alt={`Page ${p.number}`}
                         fetchPriority={isVisible ? "high" : "low"}
-                        className="w-full h-auto block [-webkit-touch-callout:none]"
+                        className="absolute inset-0 w-full h-full object-contain [-webkit-touch-callout:none]"
                         draggable={false}
                         onError={() => {
                           setImages((prev) => {