feat: Add zod validation in user.js and admin.js

[Sincha05]

##Currently, the /signup, /signin, and /google-signin routes do not validate the request body using a schema. This can lead to unexpected behavior or security issues due to malformed input.

Proposal

Integrate Zod to define and enforce schemas for the following routes:

• /signup → validate email, password, firstName, lastName
• /signin → validate email, password
• /google-signin → validate idToken

[Sincha05]

Assign this to me. I would like to solve this under GSSOC contribution

[ThakurAbhay9457]

@Sincha05 assigned