fix(chapters): address review findings + trickplay polish

Copilot + CodeRabbit review findings:
- React.memo ChapterTicks and ChapterList (project guideline: hot-path
  components must use React.memo to cut redraw work during control
  updates).
- chapterNameAt now sorts the chapter array once instead of twice per
  call. The previous version went through currentChapterIndex
  (chapterStartsMs + sort) then sortedChapters (sort again). Runs on
  every playback tick, so the duplicate work added up.
- Import getUserLibraryApi from the public barrel
  (@jellyfin/sdk/lib/utils/api) instead of the deep internal path
  (@jellyfin/sdk/lib/utils/api/user-library-api) to match the rest of
  the codebase and avoid coupling to SDK file layout.

TrickplayBubble polish:
- Sit just above the slider (bottom: 0) so the bubble no longer overlaps
  the progress bar.
- Move the chapter-name + timestamp overlay to the bottom-left of the
  preview frame, smaller font, in front of the surrounding overlays
  (zIndex + elevation).

BottomControls cleanup:
- Drop dev-only "pick one to test" comment in favour of a one-line note
  on TICK_HEIGHT.
- Inline scrubMs into its useMemo callback so the scrub-chapter-name
  lookup only recomputes while a slide is active.

components/chapters/ChapterTicks.tsx

@@ -4,7 +4,7 @@
  * so the slider underneath still receives touches.
  */
 
-import { useState } from "react";
+import { memo, useState } from "react";
 import { type LayoutChangeEvent, PixelRatio, View } from "react-native";
 import type { ChapterMarker } from "@/utils/chapters";
 
@@ -19,7 +19,7 @@ interface ChapterTicksProps {
   width?: number;
 }
 
-export function ChapterTicks({
+function ChapterTicksComponent({
   markers,
   // Semi-transparent black contrasts against both the filled progress
   // (#fff) and the unfilled track (rgba(255,255,255,0.2)) so the ticks
@@ -83,3 +83,5 @@ export function ChapterTicks({
     </View>
   );
 }
+
+export const ChapterTicks = memo(ChapterTicksComponent);