[feat]: Begin implementation of BetterAuth Sign-Up (email/password)

[henrylee746]

Description

you can start from the docs: https://www.better-auth.com/docs/installation
for now, only email/password authentication method is OK (sign-up, sign-in will be logged in a different issue)

for step 7 when setting up the mount handler, we will need to create a route in our app like such: /app/api/auth/[...all]/route.ts

for now, a simple message on the homepage or in the console if a user successfully signs up is fine. or if you want to go further, maybe a shadcn toast component that appears after a successful registration

Can implement in /app/signup/SignupForm.tsx Line 54