feat(reader): prefetch the next chapter's first pages near the end

When the reader reaches within two pages of the end of a chapter (tracked in
both modes off the furthest page reached), warm the next chapter's first few
page images via hidden imgs so flipping over renders instantly. Fetched once
per next-chapter id, guarded against mid-flight chapter changes, and retried
on failure. e2e asserts the prefetch is absent at the chapter start and
present (with the next chapter's URLs) near the end.

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

frontend/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.101.0",
+  "version": "0.102.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "mangalord-frontend",
-      "version": "0.101.0",
+      "version": "0.102.0",
       "devDependencies": {
         "@lucide/svelte": "^1.16.0",
         "@playwright/test": "^1.48.0",