fix(home): keep finished series off the Continue-reading shelf

A "Continue reading" shelf listing series the reader already finished (read
to the last page, nothing new) is odd. Expose the last-read chapter's
page_count on the read-progress list, and filter the shelf to drop entries
that are caught up (on the last page of the latest chapter with no new
chapters). Mid-chapter and has-new-chapters entries stay; the full history
view is unaffected. Finished detection is a pure, unit-tested helper.

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

backend/src/repo/read_progress.rs

@@ -141,6 +141,7 @@ pub async fn list_for_user(
                m.cover_image_path  AS manga_cover_image_path,
                rp.chapter_id,
                c.number            AS chapter_number,
+               c.page_count        AS chapter_page_count,
                rp.page,
                rp.updated_at,
                -- Personal "new since last read": how many distinct chapter