fix: resolve oxlint errors across codebase (#24)

## Summary
- Remove 5 unused `beforeEach` imports from test files
- Remove unused imports (`existsSync`, `TimelineCompositionElement`)
- Remove unused destructured variables (`options`, `width`, `height`, `goldenEl`)
- Remove dead `formatDuration` function
- Fix unused catch parameters (`catch (err)` → `catch`)
- Prefix unused `renderError` state with `_`
- Add `eslint-disable-next-line` for 2 React exhaustive-deps false positives (stable ref + zustand setter)

Part 2/4 of [VA-851](https://linear.app/heygen/issue/VA-851/pre-migration-configure-eslint-prettier-and-conventional-commits)

## Test plan
- [x] `pnpm lint` — 0 errors on 193 files
- [x] All 348 tests pass (core + engine)

packages/studio/src/player/hooks/useTimelinePlayer.ts

@@ -388,6 +388,7 @@ export function useTimelinePlayer() {
         console.warn("Could not find __player, __timeline, or __timelines on iframe after 5s");
       }
     }, 200);
+  // eslint-disable-next-line react-hooks/exhaustive-deps -- setElements is a stable zustand setter
   }, [getAdapter, setDuration, setCurrentTime, setTimelineReady, setIsPlaying, processTimelineMessage]);
 
   /** Save the current playback time so the next onIframeLoad restores it. */