feat: preload the current chapter ahead of the reader

Continuous mode eager-loaded only pages 0..=initialIndex; everything
below stayed `loading="lazy"`, so pages fetched and reflowed in as they
scrolled into view. Now a rolling window eager-loads a few pages ahead of
the reading position (PRELOAD_AHEAD) so they arrive — fetched and sized —
while still below the fold, then swap in without pop-in.

The window rolls forward off `scrollLeadIdx`, the top-of-viewport page
measured from live rects and guarded by `scrollY`. It deliberately does
not use the read-progress high-water mark: before images load they are
0-height and pile into the viewport, which would latch that mark to the
last page and eager-load the whole chapter on open. Pages past the window
stay lazy so opening a long chapter doesn't fetch every page at once.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.122.1",
+  "version": "0.123.0",
   "private": true,
   "type": "module",
   "scripts": {