Get username checks up to date with Rust branch

[strawmelonjuice]

Applies to:

• Backend
• Frontend (Gleam)
• Backend NIF (Rust)
• Legacy frontend (TypeScript)
• Legacy backend (Rust)

In the backend, username check on create-account function is NOT complete (however functional). It is also not at all implemented on both frontend and backend to perform and display username checks BEFORE submit.

This needs to be fixed!