feat(loader): DLT-2963 add track background to loader

[Francis Rupert (francisrupert)]

Screen.Recording.2026-03-16.at.11.10.26.PM.mov

🛠️ Type Of Change

• Feature

📖 Jira Ticket

https://dialpad.atlassian.net/browse/DLT-2963

📖 Description

Slight visual refresh: add a "track" for a balanced visual

📝 Checklist

For all PRs:

• I have ensured no private Dialpad links or info are in the code or pull request description (Dialtone is a public repo!).
• I have reviewed my changes.
• I have added all relevant documentation.
• I have considered the performance impact of my change.

For all CSS changes:

• I have used design tokens whenever possible.
• I have considered how this change will behave on different screen sizes.
• I have visually validated my change in light and dark mode.
• I have used gap or flexbox properties for layout instead of margin whenever possible.

[github-actions]

✔️ Deploy previews ready!
😎 Dialtone documentation preview: https://dialtone.dialpad.com/deploy-previews/pr-1131/
😎 Dialtone-vue preview: https://dialtone.dialpad.com/vue/deploy-previews/pr-1131/