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>

frontend/src/lib/components/TapZone.svelte

@@ -69,6 +69,10 @@
         const startY = e.clientY;
         pressTimer = setTimeout(() => {
             pressTimer = null;
+            // A long-press consumed the gesture — cancel any swipe so a
+            // subsequent drag-then-lift (finger held, then moved) doesn't
+            // also turn the page on top of opening the action sheet.
+            swipeStart = null;
             suppressNextClick = true;
             if (suppressExpiryTimer != null) clearTimeout(suppressExpiryTimer);
             suppressExpiryTimer = setTimeout(() => {
@@ -200,6 +204,9 @@
         padding: 0;
         margin: 0;
         cursor: pointer;
+        /* Let vertical panning through to the page but keep horizontal
+           gestures for our swipe handler rather than browser scroll. */
+        touch-action: pan-y;
         /* No visible style — these are interaction surfaces, not
            controls. Focus-visible is honored so keyboard users get
            a ring if they tab into one. */