feat: add volunteer management with SMS integration (#3)

[Imole203]

feat: Volunteer Management with SMS Integration

Summary

Implements a volunteer management system with SMS notification support for the
TeachLink platform.

Changes

src/types/volunteer.ts

• Defines Volunteer, VolunteerRole, VolunteerStatus, VolunteerSMSPreferences,
  and VolunteerSMSPayload types

src/app/store/volunteerStore.ts

• Zustand store with localStorage persistence for volunteer CRUD operations
• Actions: addVolunteer, updateVolunteer, removeVolunteer, updateSMSPreferences

src/app/api/volunteers/route.ts

• Edge-compatible GET and POST API routes for volunteer management
• Validates required fields (name, email, role) on creation

What's Next (remaining tasks)

• VolunteerManagement UI component (list, add, edit, SMS opt-in/out)
• useVolunteerSMS hook integrating with existing notification system
• Admin page at /admin/volunteers

Testing

• Types are fully typed — no implicit any
• Store follows the same pattern as notificationStore
• API returns proper status codes (201 on create, 400 on bad input)

Closes #457

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