feat: add phone number field to agent registration form

[nadavosa]

Summary

• Adds required phone input to AccountStep after the confirm-password field
• Validates minimum 7 characters (matches person.phone BE constraint)
• Sends phone inside the person object on POST /user (already accepted by BE)
• Adds agentRegistration.fields.phone and agentRegistration.errors.phoneTooShort keys to EN and DE translations

Closes #676
Depends on need4deed-org/be#685 (merged via BE PR #693)

Test plan

• Fill registration form — phone field appears after "Confirm password"
• Submit with empty phone — required error shown
• Submit with phone shorter than 7 chars — too-short error shown
• Submit with valid phone — user created, person.phone set in DB
• German locale shows correct labels and error messages

🤖 Generated with Claude Code

[ivannissimrch]

I tested it locally, and it works. The phone field appears after confirm-password; both the empty and under-7-character errors fire, and the German labels look correct. I also checked the DB, and the phone saves correctly for the person.