Fix authentication form validation and incorrect error handling

[compiler041]

Problem:
The authentication forms (login & signup) currently show incorrect and confusing error behavior:

• Generic "Failed to fetch" / "Unexpected error occurred" messages appear for client-side validation cases.
• Duplicate error messages are rendered in some cases.
• Password input becomes blocked or unresponsive when email validation fails.
• Backend/network errors and client-side validation errors are mixed together.
• Some validations trigger too early and prevent normal typing.

This results in poor UX and makes it difficult for users to understand what went wrong.

Expected Behavior

• Users should always be able to freely type email and password.
• Validation should only occur on form submission.
• Clear, specific client-side error messages should be shown for:
  • Empty email
  • Empty password
  • Short password
  • Password mismatch (signup)
• Backend errors should be displayed separately.
• No duplicate or stale error messages should persist.

Proposed Fix

• Separate client-side validation errors from backend errors.
• Run validation only on submit (not on change).
• Remove blocking validations that prevent typing.
• Show only one error message at a time.
• Clean unused variables and lint issues.

I’m working on a fix and would like to submit a PR for this.

[compiler041]

I am willing to Submit a PR