fix: resolve oxlint errors across codebase

[vanceingalls]

Summary

• Remove 5 unused beforeEach imports from test files
• Remove unused existsSync import, TimelineCompositionElement type import
• 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

Test plan

• pnpm lint — 0 errors on 193 files
• All 348 tests pass (core + engine)

🤖 Generated with Claude Code

[linear]

VA-851 [Pre-Migration] Configure ESLint, Prettier, and conventional commits

Context

The codebase currently has no linting or formatting enforcement. Code is clean by convention, but OSS contributors need automated guardrails. Team feedback: "I love when an open source repo has this."

What to do

1. ESLint — add configuration at monorepo root with TypeScript rules. Extend from @typescript-eslint/recommended.
2. Prettier — add .prettierrc at root. Configure for the existing code style (double quotes, no semicolons or with — match what's already there).
3. Conventional commits — add commitlint config enforcing conventional commit format (feat:, fix:, refactor:, etc.). This enables automated changelog generation later.
4. Run formatter across all source files to establish baseline (single "format" commit in internal repo before port).
5. Add lint/format scripts to root package.json: pnpm lint, pnpm format, pnpm format:check
6. Verify all existing code passes lint + format checks with zero errors

Why before port

We want the OSS repo's first commit to already have these configs in place and all code conforming. No "fix linting" commits in public history.