feat(detail): mark read chapters and count new ones on the manga page

On the manga detail page, dim chapters at or before the reader's last-read
chapter (with an accent check + screen-reader "Read." label) and show a
"N new since last read" badge counting chapters that have landed since.
All client-side from the already-loaded chapter list and per-manga read
progress — no extra requests. The count reflects the loaded (paginated)
chapter list. Logic lives in a pure, unit-tested chapterProgress helper;
e2e covers the mid-series and never-opened cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/chapterProgress.ts

@@ -0,0 +1,28 @@
+// Pure helpers for reflecting a reader's personal progress on a manga's
+// chapter list. Kept UI-free so the read/unread and "new since last read"
+// logic is unit-testable without rendering the detail page.
+
+export type NumberedChapter = { number: number };
+
+/**
+ * A chapter counts as read once its number is at or before the reader's
+ * last-read chapter number. An unknown last-read position (guest, never
+ * opened, or a deleted chapter) marks nothing — we never guess.
+ */
+export function isChapterRead(chapterNumber: number, lastReadNumber: number | null): boolean {
+    return lastReadNumber != null && chapterNumber <= lastReadNumber;
+}
+
+/**
+ * How many of the given chapters sit past the reader's last-read chapter
+ * (by number) — the personal "new since you last read" count. `0` when the
+ * last-read position is unknown. Counts over the provided (loaded) list, so
+ * for a paginated chapter list it reflects what's currently shown.
+ */
+export function countNewChapters(
+    chapters: NumberedChapter[],
+    lastReadNumber: number | null
+): number {
+    if (lastReadNumber == null) return 0;
+    return chapters.reduce((n, c) => (c.number > lastReadNumber ? n + 1 : n), 0);
+}