feat(frontend): add admin volunteer management shell

[Edwz208]

Summary

Adds the basic component for admin to view volunteers and reject and approve volunteer applications.

Related Issues

• Relates to #7

Changes

• Added "protected route" /admin/volunteers. Note placeholder authentication until authentication is complete
• Can approve/reject volunteer applications with live-updates
• Note that can view all volunteers but have yet to categorize them into applicants and approved

Testing

Run "npm run test" and "npm run type-check"

Screenshots / UI (if applicable)

• After:

Checklist

• Tests added or updated
• CI passes (lint, tests, build)
• Documentation updated (if behavior changed)
• No secrets or credentials committed