ci: pass CODECOV_TOKEN and add codecov.yml for PR comments (#10774)

## What

Follow-up to #10575. Pass `CODECOV_TOKEN` secret to codecov upload
action and add `codecov.yml` config so Codecov posts coverage diff
comments on PRs.

## Changes

- `ci-tests-unit.yaml`: add `token: ${{ secrets.CODECOV_TOKEN }}`
- `codecov.yml`: configure PR comment layout (header, diff, flags,
files)

## Manual Step Required

An admin needs to add the `CODECOV_TOKEN` secret to the repo:

1. Go to [codecov.io](https://app.codecov.io) → sign in → find
`Comfy-Org/ComfyUI_frontend` → Settings → General → copy the Repository
Upload Token
2. Go to [repo
secrets](https://github.com/Comfy-Org/ComfyUI_frontend/settings/secrets/actions)
→ New repository secret → name: `CODECOV_TOKEN`, value: the token

## Testing

Config-only change. Once the secret is added, the next PR will get a
Codecov coverage comment.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10774-ci-pass-CODECOV_TOKEN-and-add-codecov-yml-for-PR-comments-3346d73d36508169bac5e61eecc94063)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

.github/workflows/ci-tests-unit.yaml

@@ -31,4 +31,5 @@ jobs:
         uses: codecov/codecov-action@1af58845a975a7985b0beb0cbe6fbbb71a41dbad # v5.5.3
         with:
           files: coverage/lcov.info
+          token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: false