fix: bring back initial loading spinner in launchpad

[marktnoonan]

Closes: #24950

User facing changelog

The launchpad will now show a loading spinner on startup to indicate that it has successfully started an is a loading state.

Additional details

We had this previously but some changes to the launchpad in 10.11.0 unintentionally removed it. This PR adds the spinner in the index.html to display it as quickly as possible, before JS has even loaded and mounted the app.

Steps to test

Open and Cypress project and make sure that you see a spinner before the Testing Type picker.

How has the user experience changed?

Before - no spinner, blank screen until testing type cards appear:

After - spinner immediately on startup

PR Tasks

• Have tests been added/updated?
• Has the original issue (or this PR, if no issue exists) been tagged with a release in ZenHub? (user-facing changes only)
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?

[cypress-bot]

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[ZachJW34]

Works great!