fix(audiobooks): satisfy noUncheckedIndexedAccess in audiobook code

tsc -b runs with noUncheckedIndexedAccess on, so indexed reads must
guard for undefined even when the loop bounds make it provably safe.
Bind the indexed value to a local in findChapterAt and ChaptersSection,
and use non-null assertions in tests where the test data shape makes
the indexed element guaranteed. No runtime change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/pages/audiobooks/AudiobookDetail.tsx

@@ -39,8 +39,9 @@ function findChapterAt(
   seconds: number,
 ): { label: string; index: number } | null {
   for (let i = chapters.length - 1; i >= 0; i--) {
-    if (seconds >= chapters[i].absoluteStart) {
-      return { label: chapters[i].label, index: i + 1 };
+    const ch = chapters[i];
+    if (ch && seconds >= ch.absoluteStart) {
+      return { label: ch.label, index: i + 1 };
     }
   }
   return chapters[0] ? { label: chapters[0].label, index: 1 } : null;

web/src/pages/audiobooks/components/ChaptersSection.test.tsx

@@ -32,13 +32,13 @@ describe("ChaptersSection", () => {
   it("sort menu switches between position and longest-first orders", async () => {
     render(<ChaptersSection files={files} currentPositionSeconds={null} onSelect={vi.fn()} />);
     const rowsBefore = screen.getAllByRole("button", { name: /Prologue|Memory/ });
-    expect(within(rowsBefore[0]).getByText("Prologue")).toBeInTheDocument();
+    expect(within(rowsBefore[0]!).getByText("Prologue")).toBeInTheDocument();
 
     await userEvent.click(screen.getByRole("button", { name: /sort/i }));
     await userEvent.click(screen.getByRole("menuitem", { name: /longest first/i }));
 
     const rowsAfter = screen.getAllByRole("button", { name: /Prologue|Memory/ });
-    expect(within(rowsAfter[0]).getByText("Memory")).toBeInTheDocument();
+    expect(within(rowsAfter[0]!).getByText("Memory")).toBeInTheDocument();
   });
 
   it("calls onSelect with absolute start seconds when a chapter is clicked", async () => {

web/src/pages/audiobooks/components/ChaptersSection.tsx

@@ -68,7 +68,8 @@ export function ChaptersSection({ files, currentPositionSeconds, onSelect }: Cha
   const currentIndex = useMemo(() => {
     if (currentPositionSeconds == null) return -1;
     for (let i = rows.length - 1; i >= 0; i--) {
-      if (currentPositionSeconds >= rows[i].absoluteStart) return i;
+      const row = rows[i];
+      if (row && currentPositionSeconds >= row.absoluteStart) return i;
     }
     return -1;
   }, [rows, currentPositionSeconds]);

web/src/pages/audiobooks/player/useAudiobookPlayback.test.ts

@@ -41,8 +41,8 @@ describe("useAudiobookPlayback", () => {
       useAudiobookPlayback({ contentId: "c", files, initialPositionSeconds: 0 }),
     );
     expect(result.current.chapters).toHaveLength(2);
-    expect(result.current.chapters[0].start_seconds).toBe(0);
-    expect(result.current.chapters[1].start_seconds).toBe(300);
+    expect(result.current.chapters[0]!.start_seconds).toBe(0);
+    expect(result.current.chapters[1]!.start_seconds).toBe(300);
   });
 
   it("computes streamUrl from the first file id", () => {