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

internal/service/sandbox.go

@@ -86,8 +86,18 @@ func (s *SandboxService) Create(ctx context.Context, p SandboxCreateParams) (db.
 		}
 	}
 
+	if p.TeamID == "" {
+		return db.Sandbox{}, fmt.Errorf("invalid request: team_id is required")
+	}
+
+	// Determine whether this team uses BYOC hosts or platform hosts.
+	team, err := s.DB.GetTeam(ctx, p.TeamID)
+	if err != nil {
+		return db.Sandbox{}, fmt.Errorf("team not found: %w", err)
+	}
+
 	// Pick a host for this sandbox.
-	host, err := s.Scheduler.SelectHost(ctx)
+	host, err := s.Scheduler.SelectHost(ctx, p.TeamID, team.IsByoc)
 	if err != nil {
 		return db.Sandbox{}, fmt.Errorf("select host: %w", err)
 	}