fix(apps/next-js): align demo UI and docs

[anirudh-makuluri]

Pull Request Checklist

Please ensure that your PR meets the following requirements:

• I have read the CONTRIBUTING guide.
• I have updated the documentation (if applicable).
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Description

Updates the apps/next-js demo to better match the current product flow and UI behavior.

This PR:

• fixes the README so it reflects the current browser-based @moss-dev/moss-web demo
• adds a delete-index action to complete the create/load/query lifecycle in the demo
• surfaces retrieval time in the UI so the README and app behavior match
• adds test coverage for the new delete-index flow and retrieval-time display

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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[CoderOMaster]

@anirudh-makuluri thanks for the pr, will review it

[CoderOMaster]

Can you attach a short 1 minute video of changes done? (Like UI)

[anirudh-makuluri]

Thanks for reviewing @CoderOMaster , here's the link to the demo: https://www.loom.com/share/fdab220191a9485ab8804e3caf0dbeae

[CoderOMaster]

lgtm