chore(lint): fix pre-existing ESLint errors on main

[axisrow]

chore(lint): fix pre-existing ESLint debt on main

main (74ba67f) carries 21 ESLint errors from the themes work (#12/#13) that were
committed without a lint pass. This makes the required "Lint & Type Check" job
red for every branch off main — including PR #22, whose own diff is clean.

What changed

• bun run lint:fix closed 17/21 (stylistic: arrow-parens, indent,
  import-newlines, import-x/order + the prefer-find rewrite).
• Manual fixes for the remaining 4:
  • src/utils/powerline.ts — cachedFontStatus ??= checkPowerlineFonts()
    (prefer-nullish-coalescing; behavior identical).
  • src/tui/components/__tests__/PowerlineThemeSelector.test.ts —
    .filter(...)[0] → .find(...) (prefer-find).
  • src/utils/__tests__/renderer-regular-theme.test.ts — replaced the
    expect(...).toBeDefined() + ! pair with an explicit narrowing guard
    that throws with a message (the auto-fixer's ! is banned by
    no-non-null-assertion in this repo).

No lint rules disabled, no dependencies added.

Verification

• bun run lint — exit 0 (tsc + eslint, same command CI runs).
• bun test — full suite, single run (see PR checks).
• Diff touches only the 7 files named by the failing lint job.

Notes

Unblocks the CI gate for all open PRs based on main, notably #22 (its branch
inherits the red base; the PR's own files produce zero lint errors).

🤖 Generated with Claude Code

[axisrow]

Closing as redundant: main moved to b35114c (#21), which already landed the same lint fixes — after rebasing this branch onto the new base the diff is empty and bun run lint passes on main (verified locally). The remaining open lint-gated PR (e.g. #22) should re-run checks green against the updated base.