Fix #1871: Improve initial board setup loading UI with informative text

[VishakhGaitonde]

Title: Feat/Fix: Add informative UI to initial board setup loader (Closes #1871)

Closes #1871

Description of Change

This PR addresses the user experience issue where the application would show a dark screen with a bare spinner during the one-time 'initial board setup' process, causing user confusion.

Solution

I replaced the singular <CircularProgress /> component in the loading condition with an expanded UI block. This new screen includes the spinner along with centered, informative text (e.g., 'Setting up your personalized boards...') to explain the brief wait to the user.

Testing/Verification

Note to Maintainers: I have successfully confirmed the local development environment starts and the code compiles without errors. However, I was unable to fully test the Sign Up flow on my local machine due to a server connectivity error (likely needing local backend/DB setup).

I am confident the front-end change is correct and request maintainers to perform the final functional verification on the sign-up flow during the PR review.