feat: improve UX

[sebastiendan]

Description

This PR improves the general UX of the faucet:

• Initial loader on page load to let the app fetch the registered subnets
• Pre-selected subnets so that users by default request assets on all subnets
• Alert not only for errors but also successes so that users don't miss their successful faucet executions

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works