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

@@ -13,6 +13,7 @@
     } from '$lib/api/mangas';
     import { resyncManga } from '$lib/api/admin';
     import { chapterLabel } from '$lib/api/chapters';
+    import { isChapterRead, countNewChapters } from '$lib/chapterProgress';
     import { formatBytes } from '$lib/upload-validation';
     import { listTags, type Tag } from '$lib/api/tags';
     import type { ContentWarning } from '$lib/api/page_tags';
@@ -27,6 +28,7 @@
     import UploadCloud from '@lucide/svelte/icons/upload-cloud';
     import RefreshCw from '@lucide/svelte/icons/refresh-cw';
     import ChevronLeft from '@lucide/svelte/icons/chevron-left';
+    import Check from '@lucide/svelte/icons/check';
     import MoreHorizontal from '@lucide/svelte/icons/more-horizontal';
     import BookmarkIcon from '@lucide/svelte/icons/bookmark';
     import BookmarkCheck from '@lucide/svelte/icons/bookmark-check';
@@ -68,6 +70,12 @@
             : null
     );
 
+    /** The reader's last-read chapter number, used to mark chapters as read
+     *  and count what's landed since. `null` for guests / never-read. */
+    const lastReadNumber = $derived(readProgress?.chapter_number ?? null);
+    /** New chapters since last read, over the loaded chapter list. */
+    const newChapterCount = $derived(countNewChapters(chapters, lastReadNumber));
+
     const authors = $derived<AuthorRef[]>(manga.authors);
     const genres = $derived<GenreRef[]>(manga.genres);
     /** Deduped content warnings across the manga's pages (analysis worker). */
@@ -597,7 +605,14 @@
 
     <section aria-label="chapters">
         <div class="chapters-head">
-            <h2>Chapters</h2>
+            <div class="chapters-head-left">
+                <h2>Chapters</h2>
+                {#if newChapterCount > 0}
+                    <span class="new-chapters" data-testid="new-chapters-summary">
+                        {newChapterCount} new since last read
+                    </span>
+                {/if}
+            </div>
             {#if contentBytes === null || contentBytes > 0}
                 <span class="content-size" data-testid="manga-content-size">
                     Content: {contentBytes === null ? '—' : formatBytes(contentBytes)}
@@ -624,7 +639,12 @@
         {:else}
             <ol class="chapter-list" data-testid="chapter-list">
                 {#each chapters as c (c.id)}
-                    <li>
+                    {@const read = isChapterRead(c.number, lastReadNumber)}
+                    <li class:read data-testid={read ? `chapter-read-${c.id}` : undefined}>
+                        {#if read}
+                            <Check size={15} class="read-check" aria-hidden="true" />
+                            <span class="sr-only">Read.</span>
+                        {/if}
                         <a href="/manga/{manga.id}/chapter/{c.id}">
                             {chapterLabel(c)}
                         </a>
@@ -1009,6 +1029,40 @@
         padding: var(--space-1) 0;
     }
 
+    /* Already-read chapters (at or before the reader's last-read chapter)
+       dim so the eye is drawn to what's still unread; the check keeps its
+       accent colour as a positive "done" marker. */
+    .chapter-list li.read {
+        color: var(--text-muted);
+    }
+
+    .chapter-list li.read :global(svg) {
+        vertical-align: -2px;
+        margin-right: var(--space-1);
+        color: var(--primary);
+    }
+
+    .new-chapters {
+        color: var(--primary);
+        background: var(--primary-soft-bg);
+        border-radius: var(--radius-pill);
+        padding: 0 var(--space-2);
+        font-size: var(--font-sm);
+        font-weight: var(--weight-semibold);
+    }
+
+    .sr-only {
+        position: absolute;
+        width: 1px;
+        height: 1px;
+        padding: 0;
+        margin: -1px;
+        overflow: hidden;
+        clip: rect(0, 0, 0, 0);
+        white-space: nowrap;
+        border: 0;
+    }
+
     .pages {
         color: var(--text-muted);
         margin-left: var(--space-2);
@@ -1022,6 +1076,13 @@
         gap: var(--space-3);
     }
 
+    .chapters-head-left {
+        display: flex;
+        align-items: baseline;
+        gap: var(--space-2);
+        min-width: 0;
+    }
+
     .content-size {
         color: var(--text-muted);
         font-size: var(--font-sm);