Reject passwords with leading/trailing whitespace in registration

[Copilot]

Passwords with leading or trailing whitespace were accepted during registration, creating authentication issues when login flows handle whitespace inconsistently.

Changes

• Frontend validation (UserRegistrationForm.tsx): Reject passwords where password !== password.trim()
• Backend validation (route.ts): Return 400 with clear error for whitespace-padded passwords
• Test coverage (route.test.ts): Validate whitespace handling, including passwords with internal spaces (permitted)

// Frontend validation
if (formData.password !== formData.password.trim()) {
  newErrors.password = 'Password cannot have leading or trailing spaces';
}

// Backend validation  
if (password !== password.trim()) {
  return NextResponse.json(
    { error: 'Password cannot have leading or trailing spaces' },
    { status: 400 }
  );
}

Internal spaces remain valid (e.g., "my pass word" is accepted, " password " is rejected).

Original prompt

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