Track reading progress at the page level and resume there

Upgrades the reader's last-read tracking from {chapter} to {chapter, page}.

- ReadingProgressButton: storage is now JSON {chapter, page}; legacy
  bare-number values are read back as {chapter, page: 1}. Button label
  is unchanged ("继续阅读 · #N title") — the extra precision lives in
  the reader's first-fetch offset, not the label.
- PageReader: on mount with saved progress, seed offsetRef to
  (page - 1) so the first /api/pages call starts AT the resumed page
  instead of the beginning of the chapter. currentPageNum state is
  initialized from storage too, so the first persist write is a no-op
  that matches the saved value.
- Scroll tracker now also tracks currentPageNum (last page whose top
  has crossed above viewport top+80), and persistence writes the
  {chapter, page} pair on each change.

Known limitation: earlier pages of the resumed chapter aren't loaded
yet — a follow-up commit adds scroll-up prefetch for those.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

components/PageReader.tsx

@@ -3,7 +3,10 @@
 import { useState, useEffect, useRef, useCallback } from "react";
 import Link from "next/link";
 import { useRouter } from "next/navigation";
-import { writeLastReadChapter } from "@/components/ReadingProgressButton";
+import {
+  readProgress,
+  writeProgress,
+} from "@/components/ReadingProgressButton";
 
 type ChapterMeta = {
   id: number;
@@ -48,12 +51,30 @@ export function PageReader({
   const [pages, setPages] = useState<LoadedPage[]>([]);
   const [currentChapterNum, setCurrentChapterNum] =
     useState(startChapterNumber);
+  const [currentPageNum, setCurrentPageNum] = useState(() => {
+    if (typeof window === "undefined") return 1;
+    const p = readProgress(mangaSlug);
+    if (p && p.chapter === startChapterNumber && p.page > 1) return p.page;
+    return 1;
+  });
 
   const hiddenByScrollRef = useRef(false);
   const fetchChapterIdxRef = useRef(
     chapters.findIndex((c) => c.number === startChapterNumber)
   );
+  // Initialize offset from saved progress so the first fetch starts AT the
+  // user's last-read page — previous pages are skipped entirely
   const offsetRef = useRef(0);
+  const initialPageRef = useRef(1);
+  const offsetInitedRef = useRef(false);
+  if (!offsetInitedRef.current && typeof window !== "undefined") {
+    offsetInitedRef.current = true;
+    const p = readProgress(mangaSlug);
+    if (p && p.chapter === startChapterNumber && p.page > 1) {
+      offsetRef.current = p.page - 1;
+      initialPageRef.current = p.page;
+    }
+  }
   const loadingRef = useRef(false);
   const doneRef = useRef(false);
   // Count of pages already loaded — tracked via ref so fetchBatch stays stable
@@ -232,14 +253,20 @@ export function PageReader({
         hiddenByScrollRef.current = true;
         setShowUI(false);
       }
-      let current = startChapterNumber;
+      // Nothing loaded yet — don't overwrite the resumed-page state
+      if (pages.length === 0) return;
+      let chapter = pages[0].chapterNumber;
+      let page = pages[0].pageNumber;
       for (let i = 0; i < pages.length; i++) {
         const el = pageRefsRef.current.get(i);
         if (!el) continue;
-        if (el.offsetTop <= y + 80) current = pages[i].chapterNumber;
-        else break;
+        if (el.offsetTop <= y + 80) {
+          chapter = pages[i].chapterNumber;
+          page = pages[i].pageNumber;
+        } else break;
       }
-      setCurrentChapterNum(current);
+      setCurrentChapterNum(chapter);
+      setCurrentPageNum(page);
     };
     const onScroll = () => {
       if (rafId) return;
@@ -252,10 +279,15 @@ export function PageReader({
     };
   }, [pages, startChapterNumber]);
 
-  // Persist reading progress whenever the visible chapter changes
+  // Persist progress as user scrolls. offsetRef has been pre-seeded above
+  // so the first fetched page IS the resumed page — no scroll restoration
+  // needed.
   useEffect(() => {
-    writeLastReadChapter(mangaSlug, currentChapterNum);
-  }, [mangaSlug, currentChapterNum]);
+    writeProgress(mangaSlug, {
+      chapter: currentChapterNum,
+      page: currentPageNum,
+    });
+  }, [mangaSlug, currentChapterNum, currentPageNum]);
 
   const currentChapter =
     chapters.find((c) => c.number === currentChapterNum) ??

components/ReadingProgressButton.tsx

@@ -13,34 +13,59 @@ type Props = {
   chapters: ChapterLite[];
 };
 
+export type ReadingProgress = {
+  chapter: number;
+  page: number;
+};
+
 function storageKey(slug: string) {
   return `sunnymh:last-read:${slug}`;
 }
 
-export function readLastReadChapter(slug: string): number | null {
+export function readProgress(slug: string): ReadingProgress | null {
   if (typeof window === "undefined") return null;
   const raw = window.localStorage.getItem(storageKey(slug));
   if (!raw) return null;
+  // New format: JSON { chapter, page }
+  if (raw.startsWith("{")) {
+    try {
+      const parsed = JSON.parse(raw) as ReadingProgress;
+      if (
+        typeof parsed.chapter === "number" &&
+        typeof parsed.page === "number" &&
+        parsed.chapter > 0 &&
+        parsed.page > 0
+      ) {
+        return parsed;
+      }
+    } catch {
+      return null;
+    }
+    return null;
+  }
+  // Legacy format: bare chapter number
   const n = Number(raw);
-  return Number.isFinite(n) && n > 0 ? n : null;
+  return Number.isFinite(n) && n > 0 ? { chapter: n, page: 1 } : null;
 }
 
-export function writeLastReadChapter(slug: string, chapter: number) {
+export function writeProgress(slug: string, progress: ReadingProgress) {
   if (typeof window === "undefined") return;
-  window.localStorage.setItem(storageKey(slug), String(chapter));
+  window.localStorage.setItem(storageKey(slug), JSON.stringify(progress));
 }
 
 export function ReadingProgressButton({ mangaSlug, chapters }: Props) {
-  const [lastRead, setLastRead] = useState<number | null>(null);
+  const [progress, setProgress] = useState<ReadingProgress | null>(null);
 
   useEffect(() => {
-    setLastRead(readLastReadChapter(mangaSlug));
+    setProgress(readProgress(mangaSlug));
   }, [mangaSlug]);
 
   if (chapters.length === 0) return null;
   const first = chapters[0];
   const resumeChapter =
-    lastRead !== null ? chapters.find((c) => c.number === lastRead) : null;
+    progress !== null
+      ? chapters.find((c) => c.number === progress.chapter)
+      : null;
   const target = resumeChapter ?? first;
 
   return (