feat(home): add a Continue-reading shelf with new-chapter badges

Surface a horizontal "Continue reading" shelf at the top of the homepage,
fed by the user's read-progress history. Each card jumps back to the exact
page they left off (deep-linking `?page=N` past the first) and flags how
many chapters have landed since — the personal new_chapters_count from the
read-progress list. The shelf is fetched after the catalogue load so the
public browse path stays unauthenticated; guests get an empty list (401
swallowed) and the shelf stays hidden.

Add new_chapters_count to the frontend ReadProgressSummary type to match
the backend. Component unit tests cover href/deep-link, badge visibility,
and the no-chapter fallback; e2e covers signed-in (shelf + badge) and
anonymous (hidden).

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

frontend/src/routes/+page.svelte

@@ -22,7 +22,12 @@
         sortLabel as composeSortLabel,
         sortUrlParams
     } from '$lib/mangaSort';
+    import {
+        listMyReadProgressOrEmpty,
+        type ReadProgressSummary
+    } from '$lib/api/read_progress';
     import Chip from '$lib/components/Chip.svelte';
+    import ContinueReadingShelf from '$lib/components/ContinueReadingShelf.svelte';
     import MangaCard from '$lib/components/MangaCard.svelte';
     import Pager from '$lib/components/Pager.svelte';
     import SegmentedControl from '$lib/components/SegmentedControl.svelte';
@@ -37,6 +42,7 @@
     const PAGE_SIZE = 50;
 
     let mangas: MangaCardData[] = $state([]);
+    let continueEntries = $state<ReadProgressSummary[]>([]);
     let search = $state('');
     let sort = $state<MangaSort>(DEFAULT_SORT);
     let order = $state<SortOrder>(defaultOrderFor(DEFAULT_SORT));
@@ -301,6 +307,16 @@
         }
         await hydrateFromUrl();
         await load();
+        // Fetch the "Continue reading" shelf after the catalogue so the
+        // public browse path stays unauthenticated and unblocked. Returns
+        // empty for guests (401 swallowed), which hides the shelf.
+        try {
+            const progress = await listMyReadProgressOrEmpty();
+            continueEntries = progress.items;
+        } catch {
+            // Never let a history hiccup break the catalogue — leave the
+            // shelf hidden.
+        }
     });
 
     // Track viewport in a separate $effect so the listener cleans up on
@@ -455,6 +471,10 @@
     </a>
 </div>
 
+{#if continueEntries.length > 0}
+    <ContinueReadingShelf entries={continueEntries} />
+{/if}
+
 <form
     onsubmit={onSubmit}
     action="javascript:void(0)"