fix(reader): harden swipe/long-press coexistence and next-chapter prefetch

- A long-press now cancels any pending swipe, so a hold-then-drag gesture
  opens the action sheet without also turning the page (regression-tested).
- The next-chapter prefetch no longer retries on every page flip when the
  endpoint fails (guard clears only when the next chapter changes), and it
  clears a previous chapter's warmed images on navigation so stale hidden
  <img>s don't linger.
- Tap zones set touch-action: pan-y so vertical panning stays native while
  horizontal swipes reach our handler.

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 = "0.104.0"
+version = "0.105.0"
 dependencies = [
  "anyhow",
  "argon2",