feat: preload the whole current chapter in continuous mode

Eager-load every page of the current chapter rather than a bounded
rolling window, so pages are fetched up front (the browser paces them
over its per-origin connection limit, top-to-bottom = reading order)
instead of filling in as they're scrolled onto. This drops the
scrollLeadIdx/eagerThrough machinery in favour of a plain
`loading="eager"`.

The first few pages of the *next* chapter are already warmed when the
reader nears the end of the current one (the next-chapter preloader), so
flipping over stays instant.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.123.0",
+  "version": "0.124.0",
   "private": true,
   "type": "module",
   "scripts": {