fix: align testing spinner icon in start button

[mdfarid01]

Fixes #182

What changed

• Added a scoped CSS fix for the spinner icon in the "Testing" state (#startButton .btn-testing .fa-spin).
• Set letter-spacing: 0, transform-origin: 50% 50%, and vertical-align: middle to keep rotation visually centered.

Why

• The spinner appeared off-center/wobbly during the Testing state.
• This keeps the fix local to the start-button spinner and avoids affecting other icons globally.

Testing

• Ran npm run build -- staging
• Verified locally that the spinner appears centered while rotating during Testing state

[Copilot]

Pull request overview

This PR addresses UI polish in the speed test start button by scoping a CSS adjustment to the Font Awesome spinner shown during the “Testing” state, eliminating the off-center/wobbly rotation reported in #182.

Changes:

• Added a targeted CSS rule for #startButton .btn-testing .fa-spin to improve the spinner’s perceived centering while rotating.
• Adjusted letter-spacing, transform-origin, and vertical-align specifically for the “Testing” spinner to avoid global icon side effects.

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[robertodauria]

LGTM. Thanks.