fix: reader loads the full chapter list for prev/next and the dropdown

The reader fetched only the first 200 chapters, so a deep chapter (#250) fell
outside the window and its prev/next chevrons resolved to null — a dead end —
and the chapter dropdown was truncated. Add listAllChapters, which pages
through the API's 200-row cap, and use it in the reader loader.

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

frontend/src/lib/api/chapters.test.ts

@@ -9,6 +9,7 @@ import {
 } from 'vitest';
 import {
     listChapters,
+    listAllChapters,
     getChapter,
     getChapterPages,
     createChapter,
@@ -142,6 +143,30 @@ describe('chapters api client', () => {
         });
     });
 
+    it('listAllChapters pages through the 200-row cap and returns every chapter', async () => {
+        const mk = (n: number) => ({ ...chapterFixture, id: `c${n}`, number: n });
+        const first = Array.from({ length: 200 }, (_, i) => mk(i + 1));
+        const second = Array.from({ length: 50 }, (_, i) => mk(i + 201));
+        fetchSpy
+            .mockResolvedValueOnce(ok({ items: first, page: { limit: 200, offset: 0, total: 250 } }))
+            .mockResolvedValueOnce(ok({ items: second, page: { limit: 200, offset: 200, total: 250 } }));
+
+        const all = await listAllChapters('m1');
+        expect(all).toHaveLength(250);
+        expect(all[249].number).toBe(250);
+        expect(fetchSpy.mock.calls[0][0]).toContain('offset=0');
+        expect(fetchSpy.mock.calls[1][0]).toContain('offset=200');
+    });
+
+    it('listAllChapters stops after a short final page (single request)', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok({ items: [chapterFixture], page: { limit: 200, offset: 0, total: 1 } })
+        );
+        const all = await listAllChapters('m1');
+        expect(all).toHaveLength(1);
+        expect(fetchSpy).toHaveBeenCalledTimes(1);
+    });
+
     it('getChapterPages unwraps the {pages} envelope into the array', async () => {
         fetchSpy.mockResolvedValueOnce(
             ok({