Add BYOC page, admin section, and is_byoc team visibility gating

- Frontend: BYOC hosts page (/dashboard/byoc) with register/delete flows,
  shimmer loading, pulsing online status, animated token reveal checkmark
- Frontend: Admin section (/admin/hosts) with platform + BYOC tabs, stat
  pills, skeleton loading, slide-in animations for new rows
- Frontend: AdminSidebar component with accent top bar and admin pill badge
- Frontend: BYOC nav item shown only when team.is_byoc is true (derived
  from teams store, not JWT); disabled for members
- Frontend: Admin shield button in Sidebar, visible only to platform admins
- Backend: is_admin in JWT claims + requireAdmin middleware (DB-validated)
- Backend: is_byoc added to teamResponse so frontend derives visibility
  from fresh team data rather than stale JWT fields
- Backend: SetBYOC admin endpoint (PUT /v1/admin/teams/{id}/byoc)
- Backend: Admin hosts list enriches BYOC entries with team_name
- Host agent: load .env file via godotenv on startup

db/queries/hosts.sql

@@ -72,8 +72,9 @@ SELECT * FROM hosts WHERE id = $1 AND team_id = $2;
 -- Returns all hosts that have completed registration (not pending/offline).
 SELECT * FROM hosts WHERE status NOT IN ('pending', 'offline') ORDER BY created_at;
 
--- name: UpdateHostHeartbeatAndStatus :exec
+-- name: UpdateHostHeartbeatAndStatus :execrows
 -- Updates last_heartbeat_at and transitions unreachable hosts back to online.
+-- Returns 0 if no host was found (deleted), which the caller treats as 404.
 UPDATE hosts
 SET last_heartbeat_at = NOW(),
     status            = CASE WHEN status = 'unreachable' THEN 'online' ELSE status END,