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/lib/api/read_progress.ts

@@ -17,6 +17,10 @@ export type ReadProgressSummary = {
     chapter_number: number | null;
     page: number;
     updated_at: string;
+    /** Chapters past the reader's last-read chapter — a personal "new
+     * since you last read" count. `0` when caught up or when the position
+     * is unknown (manga-level progress / deleted chapter). */
+    new_chapters_count: number;
 };
 
 export type ReadProgressPage = {