Fix return type union in signup action function

[coderabbitai]

A static analysis tool (Biome) has identified that the return type Promise<Error | void> in the signup action function is confusing and should be changed to Promise<Error | undefined>.

Location: apps/web/src/app/signup/action.ts line 8

Issue: The void type in a union type is confusing according to the linter rule lint/suspicious/noConfusingVoidType.

References:

• PR: Refactor UI components and enhance Supabase integration #65
• Comment: Refactor UI components and enhance Supabase integration #65 (comment)

Reporter: @daemon-node-byte