Fix iOS freeze when editing phone number

[zaelgohary]

Summary

The phone-edit dialog was calling setState() from inside the IntlMobileField validator. With AutovalidateMode.onUserInteraction (the package default), the validator runs on every keystroke during the form's build phase — calling setState() there schedules a rebuild while a build is in progress, which on iOS manifests as a screen freeze the moment the first digit is typed.

Related Issue

Closes #1214

Changes

• Made the validator pure; moved state mutation into onChanged.
• Derived _isValid from verificationPhoneNumber.isNotEmpty and reused a _isValidPhone helper across the validator and onChanged.
• Guarded the per-keystroke setState so it only fires when the validity state actually changes.

Test Results

Tested on iPhone (iOS 26.4.1) — no freeze, can type a full number and submit.