feat(notifications): require mobile number when SMS method selected

[saewitz]

Summary

• When staff selects "Phone" or "Phone and Computer" as their notification method, the Mobile field is now marked as required
• Clicking "Save changes" without a phone number turns the field red with "Required" helper text (previously showed a generic snackbar that was easy to miss)
• Renames the field label from "Phone" to "Mobile"
• Clears the field error as soon as the user starts typing or switches to a non-phone method

Closes OTR-1875

Test plan

• Go to My Profile → Notification Settings
• Enable task or telemed notifications and set "Notify me by" to "Phone" or "Phone and Computer"
• Confirm "Mobile" field shows a required asterisk (*)
• Click "Save changes" with the Mobile field empty — field should turn red with "Required" helper text, save should not proceed
• Enter a valid phone number, click "Save changes" — should save successfully
• Set method to "Computer" — Mobile field should disable and the red error (if present) should clear
• Start typing a phone number while the field is red — error should clear immediately

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