Add admin users + admin talks review dashboard

[claudinethelobster]

Summary

• add admin role support via profiles.is_admin (migration + index)
• add RLS policies so admins can view and update all talk_submissions
• add reusable checkIsAdmin() helper
• add admin-only page at /admin/talks to review all submissions, filter by status, and update status
• add admin navigation entry in account menu for admin users
• document admin setup in README

Validation

• bunx tsc --noEmit
• bun run lint (passes; only existing warnings in app/layout.tsx)

Notes

• this app is statically exported, so admin actions are done directly via Supabase client and protected by RLS policies
• to grant admin access, set profiles.is_admin = true for a user

[samholmes]

Replaced by #99