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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.122.1"
+version = "0.123.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.122.1"
+version = "0.123.0"
 edition = "2021"
 default-run = "mangalord"