Add input validation to server actions for security

[coderabbitai]

Description

Server actions handling sensitive user data (email, password) currently lack proper input validation, relying only on client-side validation which can be bypassed.

Affected Files

• apps/web/src/app/signup/action.ts
• apps/web/src/app/signin/action.ts
• apps/web/src/app/reset/action.ts

Security Concern

The current implementation uses type casting without validation:

• Email format validation is missing
• Password complexity requirements are not enforced server-side
• Input sanitization is not performed

This creates a security vulnerability where malformed or malicious input could be processed.

Context

This issue was identified during code review and acknowledged with a comment in the code stating 'in practice, you should validate your inputs' but the validation is not implemented.

References

• PR: Refactor UI components and enhance Supabase integration #65
• Comment: Refactor UI components and enhance Supabase integration #65 (comment)
• Requested by: @daemon-node-byte