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/cli/src/commands/doctor.ts

@@ -1,6 +1,5 @@
 import { defineCommand } from "citty";
 import { execSync } from "node:child_process";
-import { existsSync } from "node:fs";
 import { c } from "../ui/colors.js";
 import { findBrowser } from "../browser/manager.js";
 import { findFFmpeg } from "../browser/ffmpeg.js";