ci: rotate AWBOT_GH_TOKEN for Dependabot Auto-merge

Master CI is currently red because the `Dependabot Auto-merge` workflow cannot authenticate.

Failed run: https://github.com/ActivityWatch/aw-webui/actions/runs/25300243380

Relevant log line:

```txt
An unexpected error occurred: Bad credentials, HttpError: Bad credentials
```

The workflow is `.github/workflows/dependabot-automerge.yml` and it passes `secrets.AWBOT_GH_TOKEN` into `ridedott/merge-me-action@v2` as `GITHUB_TOKEN`. The adjacent master runs at the same SHA (`444ed4ff`) show the screenshot comment workflow succeeded, and this failure happens before any app build/test code runs.

Action needed from someone with repo secret/admin access: rotate `AWBOT_GH_TOKEN` to a valid ActivityWatchBot token with whatever scopes the auto-merge action needs to query branch protection and squash-merge Dependabot PRs, or deliberately disable/replace the auto-merge workflow if we no longer want this path.

This is a credentials/infrastructure failure, not an aw-webui code regression.