fix: lazy-load continuous-reader pages beyond an eager window

Continuous mode rendered every page (up to 2000) with loading="eager",
fetching and decoding the whole chapter up front — enough to OOM a mobile
tab on a long chapter. Eager-load only a leading window ([0..=eagerThrough],
covering the deep-link target + a lead, min 6 pages) and lazy-load the rest;
the browser fetches them as the reader scrolls near. Unloaded lazy pages
reserve height (page width x ~1.4) so they stay distinct scroll targets for
the progress IntersectionObserver, cleared on load.

Keeps the DOM dense so the scroll-to-?page=N effect, progress observer, and
next-chapter preload trigger are all unchanged. Rewrote reader-preload-window
spec to assert the eager/lazy attribute partition + deep-link scroll landing.

Bump to 0.124.2.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.1",
+  "version": "0.124.2",
   "private": true,
   "type": "module",
   "scripts": {