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>

frontend/src/lib/continueReading.ts

@@ -0,0 +1,20 @@
+import type { ReadProgressSummary } from '$lib/api/read_progress';
+
+// Whether a read-progress entry has nothing left to continue: the reader is
+// on the last page of the latest chapter and no newer chapters exist. Such
+// finished series are kept off the homepage "Continue reading" shelf (the
+// full history view still shows them).
+export function isCaughtUp(
+    e: Pick<
+        ReadProgressSummary,
+        'new_chapters_count' | 'chapter_number' | 'chapter_page_count' | 'page'
+    >
+): boolean {
+    return (
+        e.new_chapters_count === 0 &&
+        e.chapter_number != null &&
+        e.chapter_page_count != null &&
+        e.chapter_page_count > 0 &&
+        e.page >= e.chapter_page_count
+    );
+}