feat: add phone number field to agent registration form

[nadavosa]

Sub-issue of #675.

Context

Registration (Stage 1 at /register/agent) only collects name, email and password. Phone is never stored. Without it the opportunity creation form at /dashboard/opportunities/new cannot pre-populate the contact phone field.

Blocked on

need4deed-org/be#685 — phone must be added to the registration schema and written to person.phone on the BE first.

Fix

File: src/components/AgentRegistration/steps/AccountStep.tsx

• Add a required phone input field
• Add phone to AgentRegistrationData type and defaultAgentRegistrationData in types.ts
• Add phone validation in validateStep (helpers.ts)
• Include phone in the POST /user body in AgentRegistration.tsx (confirm field name with BE feat: Opportunities table, green "matched" badge and matched volunteer name #685)