[ENHANCEMENT] Add phone number validation to user input

Describe the current behavior

Currently, there is email validation in place, but there is no comparable validation for phone numbers. Users can enter arbitrary characters without any checks to ensure the input is a valid phone number.

Describe the desired improvement

Implement phone number validation:

• Accept common formats (including country codes, e.g., +49 170 1234567).
• Ensure only valid digits and allowed special characters (+, spaces, dashes) are accepted.
• Apply validation on the client-side, similar to the existing email validation.

Why is this improvement important?

A valid phone number ensures higher quality and correctness of resume data. It prevents invalid or unusable phone numbers from being included in generated resumes.

Alternatives Considered

Additional context

See related issue for email validation: #8 (closed)