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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.124.1"
+version = "0.124.2"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.124.1"
+version = "0.124.2"
 edition = "2021"
 default-run = "mangalord"