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/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -973,11 +973,18 @@
 
     $effect(() => {
         const nc = nextChapter;
+        // Drop a previous chapter's warmed images once the next chapter
+        // changes (e.g. after navigating), so stale <img>s don't linger.
+        if (preloadedForChapterId !== null && (!nc || preloadedForChapterId !== nc.id)) {
+            preloadedForChapterId = null;
+            nextPreloadUrls = [];
+        }
         if (!nc || pages.length === 0) return;
         if (pages.length - furthestPage > PRELOAD_WITHIN_PAGES) return;
         if (preloadedForChapterId === nc.id) return;
-        // Guard set before the await so overlapping effect runs don't
-        // double-fetch; cleared on failure so a later pass can retry.
+        // Marked before the await and NOT reset on failure, so a broken
+        // endpoint can't re-fetch on every page flip near the end — the
+        // guard clears only when the next chapter itself changes (above).
         preloadedForChapterId = nc.id;
         getChapterPages(manga.id, nc.id)
             .then((ps) => {
@@ -987,7 +994,7 @@
                     .map((p) => fileUrl(p.storage_key));
             })
             .catch(() => {
-                if (preloadedForChapterId === nc.id) preloadedForChapterId = null;
+                // Best-effort prefetch — swallow and don't retry this chapter.
             });
     });