feat(unlock-app): better loading UI for the Wallet-less registration block

[julien51]

Description

Increasing the side of the loader, but also its contrast. This will affect all Placeholders in the app!

Checklist:

• 1 PR, 1 purpose: my Pull Request applies to a single purpose
• I have commented my code, particularly in hard-to-understand areas
• I have updated the docs to reflect my changes if applicable
• I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
• I have performed a self-review of my own code
• If my code involves visual changes, I am adding applicable screenshots to this thread

Release Note Draft Snippet

[SVell]

Looks good